Karrera Coupe

I am struggling to diagnose the cause of a droning noise in my car, a 140,000 mile 1996 968 Coupe. The noise has already lead me to replace the front wheel bearings on my car but that had no impact on the noise.

My car has Continental Extreme Contact front tires that are about 30,000 miles and three years old with good thread remaining. The rear tires are Firestone Firehawk Indy 500 that are just two weeks old. I bought the Firestones as I was unable to source new 17” Continentals and my car was due for its annual inspection. I had the front and rear wheels aligned when I had the new rear tires installed.

I am not sure when the droning started. After an incident during a very long road trip up to Canada, my car was very noisy and the engine ran with a lot of vibration. Both the balance shafts were out of sync. With considerable help from this Forum and Clarks Garage I realigned the balance shafts. The vibration and engine noise disappeared. I immediately also had the rear tires replaced. Only after all of this did I became aware of this droning noise.

This past weekend I took the car for quite a long drive in the country. I pushed it a little at times.

The droning noise seems to be coming from the mid to rear of the car and is perhaps more noticeable on the right hand side (passenger side) of the car.

Here are my observations re. the droning sound.

1. It starts at around 40 mph and peaks at around 55 mph. It may actually decline at speeds over 65 mph. I certainly did not hear it at over 75 mph, but then at that speed there is considerable more wind noise that may be camouflaging the droning noise. Closer to the lower end of the band, i.e. 40 mph the sound seems to be deeper (more bass) and the pitch goes slightly higher (more treble) as speed increases.

2. It does not change with changes in RPM nor does the particular gear the transmission is in, have any effect.

3. Neither acceleration nor deceleration while the transmission is in gear, have any impact.

4. Engaging the clutch or releasing the clutch has no impact on the sound.

5. Whether the transmission is in neutral or in gear has no impact.

6. Swerving the car to left and right may have a slight impact on the pitch of the droning sound but there is not really discernable change in the sound.

7. After a spirited 30 mile drive, none of the wheels gave off discernably more heat than any of the others. In fact all four wheels were more or less the same temperature and only warm to the touch, certainly not scolding hot.

8. I jacked up the car and checked the 'play' in the rear wheels by placing my hands first at 12 & 6 o'clock and then at 9 and 3 o'clock and tried to rock the wheels by pushing and pulling. There is no play at all in either of them.

9. Both rear wheels are pretty tight. They do both turn but the right rear right wheel is harder to turn by hand than the left rear wheel. The right wheel stops very soon after I stop turning it by hand whereas the left rear wheel keeps rolling a little after I stop turning it by hand. This may well just be from residual resistance from the brake pads. But I do think the droning sound is more likely coming from the right side of the car.

While inspecting the car I noticed that the following:

1. The left front ball joint boot is ripped.

2. The left axle inner CV joint boot (above the exhaust) is cleanly split right round the boot. The other three boots are intact.

On the PPI report that I had done when I bought the car three years ago the following items have not yet been addressed:

1. “Right front strut is starting to leak”.

2. “Rear Shocks have noise under compression”.

3. “Ball Joint Boot cracked on driver front control arm”.

4. “Service of axles recommended”.

I do not have the early maintenance records for the car. But I do have an invoice for the replacement of the right rear wheel bearing in 2001 at 101,000 miles.

My car does not have its original engine nor transmission. Both current engine and transmission have considerably less miles on them than the car.

The current transmission does not include a LSD.

A very knowledgeable friend (he is a renowned restorer of 911 long hoods) has driven the car a couple of times with me. He first thought that it is the tires that are causing the droning sound but when I told him that the rear tires are brand new he suggested that I replace the rear wheel bearings as my next step. I only subsequently found out that the right rear wheel bearing was replaced in 2001.

My own thinking is that I should service the axles first and if that does not solve the problem then replace both the rear wheel bearings. I would have liked to try the car with different rear tires but I am not sure how to achieve this other than to buy another set. By the way I just noticed that Tire Rack seems to have the Continentals in stock in again.

All opinions and advice as to what may be the cause and remedy of my droning noise problem will be much appreciated and likely to save me a lot of money and effort!

Thanks in advance.

Alwyn

JET951

Rear wheel bearing/s , one side is probable worse than the other , plus we have seen plenty of new & old tyres( tires) that are inherently noisy , I know the continentals are generally not noisy , but the rears you have I have no experience with them & yes when tyres ( tires) are noisy they often sound like worn wheel bearings

What we do to check rear wheel bearings ( if suspect ) on a hoist

A ) We remove both rear drive shafts

B ) We remove the rear brake pads ( there is a reason for this )

C ) Put the rear wheels back on & with the rear pads out there will be no drag ( meaning you can spin up one rear wheel by hand easily ) , we then spin one rear wheel at a time ( by hand ) and listen to the rear wheel bearing , if a rear wheel bearing is noisy on the road , this will show it up , naturally you do not want any background noise , so doing this simple test is only valid if the area you are doing it is quiet , its very noticeable when you compare one side to the other

jsheiry

I noticed that when the rear wheel bearing were bad in my wifes car the noise would get lower/less pronounced when brakes applied. Effectively the pads/rotors causing the bearings to "stand up" straight momentarily.

Karrera Coupe

Thank you John and Jet951 for weighing in.

The braking test is one driving test I have not yet done!. As soon as we get a dry day I will go for another drive and try it out.
After that I will go the route Jet951 suggested.
Should I foresee any issues arising when I disconnect the two rear half shafts from the hubs? I am wondering about being able to reuse the cheesehead bolts, etc. Maybe I should address the one torn CV joint rubber boot as well as re-grease the joints while I attend to the shafts . I have just received the long 8mm cheesehead wrench socket that I ordered for this job, I will go ahead and order a CV boot. Can I assume my car has the 25 splines? It looks like I need to be sure before ordering the CV boot kits from 944online.com

Hopefully this is all within the scope of my very limited wrenching experience and skills.

Thanks again for your advice.

Any other opinions, experience and tips?
Alwyn

Yogii

I have the Firestone Indy 500 tires all around and like them alot. I do not believe that they are the cause of your noise.
Hope you find it!

-Yogii
AKA 968 Novice

Karrera Coupe

Thanks Yogii. Your comments are very helpful. I will move the tires issue further down my on my probable causes list.
Cheers

Yogii

How loud is the droning? Is it so loud that conversation is impacted?

-Y

Karrera Coupe

It is not easy to describe how loud it is. One can still carry on a verbal discussion with a passenger but it certainly does impact trying to have a phone conversation over a speaker phone that is not held to the ear. It also impacts on the enjoyment of good music in the car.
Generally our cars are quite loud. Mine does not have the correct windscreen (my car is a coupe but has a cab windscreen installed) and I think that may cause more wind noise at speed than would be the case if it had the correct windscreen.
ut the droning is noticeable as a low (bass) noise and three different passengers have confirmed their awareness of it once I pointed out to them when it starts. I am not sure they would have heard it without me pointing it out to them.
I admit that my OCD is getting worse with age! But I would like to think that my urgency in finding the cause and a remedy does have more to do with my fear that some crucial component is wearing out and maybe on its way to failing when I am far from home. My wife and I would like to drive up to Indiana in the spring but we are a little skittish after our break down in Canada this past October.
Thanks for your interest.

sublm8

I would suggest investigating the parking brake mechanism on the right. Has this been adjusted recently? It may be set too tight and the P-brake shoes may be rubbing on the right.

I had a spring come loose on my P-brake making a very alarming sound. It took multiple trips to the shop before I figured it out.

Karrera Coupe

Thanks sublm8. I will take a good look at the parking brake situation. I have not recently made any adjustments to the parking brake. In fact it is one of the items on my 968-to-do-list - the parking brake handle is not stiff enough. One has to pull it up high before it becomes effective. I also have to replace the parking brake warning light switch that sits under the handle. So maybe something is going one with the parking brake, although I would have thought it would generate a sound right through the speed range rather than just start at around 40 mph.
Thanks again.

Charlie

Make sure the rear hatch is sealing good . I was getting a lot of noise and noticed a small gap between the hatch and seal. The gap was the thickness of a business card on the passenger side.

Karrera Coupe

Charlie I have had that same problem with my hatch, I recently replaced the seal on my hatch and it now opens a lot quicker using the electric release but it still sometimes do not close properly leaving a narrow opening also on the rear passenger side of the hatch. Unfortunately closing it properly (using a bit more force) does not reduce my droning problem.
Later this week I am planning to get the car up on a rack and to use a mechanic's stethoscope while it is running to try and identify the cause. I will give feedback how that goes.
Thanks for your input.

Karrera Coupe

John there is no change in my droning noise when applying the brake. The only determining factor is still the speed, i.e. between 40 mph and around 60 mph.

Earlier today I took my car over to a local independent shop that specializes in Mercs, BWM and Porsche. They put it up on a lift with a driver in the cabin. He put power to the rear wheels. While the rear wheels were turning a mechanics stethoscope revealed that there are more sound coming from the left rear wheel hub than from the right rear wheel hub - the right rear wheel hub was really very quiet. This is a little strange as it sounds as if the droning is coming from the right side of the car. On the other hand it does make sense for the left rear bearing to be deteriorating as the maintenance records shows a right rear wheel bearing was the replaced in 2001 or about 40,000 miles ago. Through the stethoscope the transmission itself and the torque tube were much noisier than either of the ear wheel hubs.

None of the technicians in the shop thought that the left rear wheel bearing's failure is imminent. They considered the sound to be low key enough that they think it is good for a few thousand more miles before I should seriously consider replacing that rear left wheel bearing.

So after all of this I am still not much wiser as to what is causing the droning sound. I am now starting to wonder whether I am overly sensitive to this droning sound and that I should just try to ignore it.

would really like to be able to compare it to another 968 but unfortunately I do not know any local 968 owners that I can go and compare my car with.

Do the experienced 968 owners out there have any other ideas for me?

Thanks in advance.

thomasmryan

bearings, among other things, are usually replaced in pairs.

procedures should be followed as noted in the manual and standard practices...such as using a new axle nut. some manufactures specify tightening the axle bolt un-laden or only compressing a hydraulic tensioner in the vertical position.

Karrera Coupe

Thanks Thomas
I am in the dark as to what "tightening the bolt un-laden" and "only compressing a hydraulic tensioner in the vertical position" mean?
I see your location is in the Smokey Mountains. Is that in North Carolina?
Regards
Alwyn