Well I noticed this engine rattle only in the range from 600-850 rpms. I traced it down to the front passenger side of the engine compartment and after doing some research I found some info about it possibly being the AC compressor. Under load (car in drive with foot on the brake) I can get it to repeat the rattle ever single time in the RPM range stated, with the AC OFF. But the instant I turn the AC on the noise goes away completely. Has anybody had any experience with this? I read something about a software updates to cure this on a 545 but wasnt sure if it was the same for my 525? The noise is definitely coming from lower down near the AC pulley, could the pulley or AC clutch be bad or would it need a whole new compressor? BTW the AC still works flawlessly.

 

been doing some indepth research on this and found a few threads of older 5 series (pre04) that a similar issue, does anybody know if the same ac compressor is used on both models?