A different exhaust is, many times, not an "upgrade." And yes, aftermarket exhaust systems can often make unusual noises; your PPI tech should have explained that to you, perhaps pointing out a stress crack, a blown gasket, etc. More things than the exhaust can cause popping, unfortunately, with Turbos, they're not all the same, and virtually impossible to diagnose in a forum like this. A second opinion might serve you well...
Pete
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