RESONANCE
 

My 2005 X3 2.0d SE is nice and smooth and quiet for the first few miles after a cold start up.

But as it warms up, it becomes noisier and rough under 2000rpm, and a resonance develops which can be a bit unpleasant at times. Current Mileage is 30000.

Be grateful for any ideas as to what may be causing this.:dunno:

by resonance do you mean a metallic vibrating noise? Mine is starting to do that and it is the flapper valve on the muffler. Common problem with the x3