'08 535xi with 150,000km. When I'm in 'stop and crawl' traffic I hear a moaning sound that rises and falls in pitch with engine speed, say 1500-2000 RPM. If I turn off the AC the sound goes away. Narrowed it down after finding the noise disappeared when turning off the HVAC entirely (by turning the fan speed knob all the way down, then again after the lowest speed setting) - originally thought it might be the cabin fan speed varying with alternator voltage.

AC works fine for its own part but has one other oddity: sometimes on hot days it takes a minute before it starts blowing cold, usually after being parked for hours. Wonder if the compressor is on the way out.

 

You are probably low on freon. The AC compressor is probably ok, but I would not keep on running the AC with very low freon for it also has lubricant to lube the compressor. Some compressors gets burned out that way.

Fill the freon up and check for leaks. Once filled to normal, the noise should go away.

 

My 08 535 does the same, maybe not a moaning noise, but clearly a strange noise. I don't notice it too much anymore, but it's still there. I have no idea what it is either, kinda thought the compressor was getting ready to go, but I know its been doing this for two summers.

 

Interesting. Next time I'm in getting raped at the stealer I'll have them check. Thanks.

 

My compressor is making noise as well. Most likely we all need to have the compressor replaced or rebuilt. But, compressors have been known to live long lives even after they get a bit noisy. Mine is more of whine than a groan. I plan on having the compressor rebuilt this winter.

But, as 10550i suggested, there is no harm in seeing if low refrigerant level is the cause of the noise. Don't just top off R134a refrigerant, remove and replace with the exact weight of refrigerant called for on the green label under the hood.

 

Well, perhaps more a woooo sound than a moan, but I know what you mean. And I've got lots of reefer experience - would never just 'top it up' without the right instruments, a proper evac and the right charge.