Interesting - I did wonder about that. I originally thought that the drone was related to the design of the exhaust system and the materials it's made from. I have now come to the conclusion that the car - location of the engine, seating compartment, etc are susceptible to reverberation from the exhaust sound i.e. it doesn't matter what system you use - when the exhaust noise reaches a certain level you get the drone. This seems to be in line with your experience as well. Maybe this is why the Boxster exhaust is so quiet in std form...
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2003 Boxster S
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