I've got a 2005 cooper convertible which started to sound louder a few weeks ago. Took it to my mechanic who took a look and found there was a small hole in the muffler. I figured it was a good time to upgrade the muffler so I ordered the magna flow post cat system and had him install it. After installing it the exhaust was much louder and deeper but there were also other sounds/vibrations coming from under the car which you only heard if moving and going past concrete walls. It was louder from under the drivers side.
Went back to the mechanic today and he adjusted the exhaust system to increase clearance from the exhaust shield. There was also a bolt missing on one part of the exhaust shield which he fixed.
Went for a drive with him and we can still hear a sound from under the car near the front and more from the drivers side. Can't hear anything unusual if not moving and just rev the engine. He checked the exhaust and did not note any other leaks and found the engine mounts were fine. He was wondering if my KandN air filter was just causing more intake noise but I don't think so. It is more noticeable when decelerating and decreases when accelerating.
Any ideas? Could it be the flexible pipe just before the cat? Could there be a tiny hole in it?