Drone/Rumble Noise
 

2010, A6 3.0 Supercharged. I have a noise that sounds like an exhaust leak. I can only hear it from about 2,400 RPM to about 2,800 RPM when the engine is under torque (accelerating) or on deceleration through that same RPM range. I took it to a muffler shop and the guy (who seemed competent) says he's convinced that there is no exhaust leak. He agreed that it sounds like an exhaust leak but he has no idea what it could be. There also seems to be a bit of vibration (like a resonance). It happens in all gears at that RPM. The noise level is not huge but definitely noticeable. When in neutral, it's barely perceptible if at all.

Any thoughts?

Thanks,

Rick