I have a 02 525i. While in gear and accelerating around 2000 RPMs, there is an intermittent resonation or buzzing/grinding noise from under the car/engine. It happens more often going up an incline.

The BMW folks have tried 4 times to fix it without success. They felt it was an air/vacuum issue and replaced the belts, pulleys, brake boost, intake valves and other stuff but no luck.

Any idea what this could be???