I have had this low pitched whining sound coming from the right side of the engine bay for quite some time now. It happens on and off and usually at speeds up to 40 MPH. At first I assumed it relates to the tranny since it is RPM dependent, but it never happens with the AC off. Since during the winter months I mainly kept the AC off I did not hear that sound. On extended road trips during last summer and the AC always on automatic, the sound did not appear for days and then suddenly it was back

No error codes show up and both the AC and the shifting seem to work flawlessly.

 

Yeah, my '07 525i makes noise too! Seems worse when ambient temps are 75 and below. I think the compressor is on its way out!

 

If your noise isn't there when the AC is off... Then look at the AC Compressor.

 

Is it possible what you are hearing is the whining of thepower steering? possibly low on fluid? I recently had that and my fluid was low. Filled and went away.

 

Mines the a/c compressor. You can turn it off and it goes away. When temps are around 80 or above, you don't hear it. Only when it's 75 or below. Weird!

 

Same here. Will watch the temperature range to verify.

 

I'm wondering if it could be low on freon. Think I'll get it checked.

 

Yesterday, for the first time this year, did the outside temps rise into the mid 80's and no whine. Before, we had temps in the 70's and I got on and off whining.
I'll keep watching and possibly get the freon level checked.

 

I have heard variable displacement compressors on other makes whine when the compressor is at lower displacement operation in cold weather.

 

Is there a fix for that, or is it OK to live with it?