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The fart noise or "burble" like BimmerZealot mentioned is not abnormal. It's there on a stock Z4 but so quiet that you can barely hear it. On my '04 Z4 3.0 with ACS exhaust, the burbles were much louder. On my Z4M with Eisenmann sport now the burbles are not as pronounced.
As for the 6% performance gain, the guy was either clueless or simply lying to make a sale. Your Z4 actually just lost some torque and is slower than before. The difference is probably not big enough for you to notice it though.
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2019 Porsche 911 4 GTS
2016 BMW M3
2008 BMW Z4M coupe
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