bronto

Seems to be the air pump.

Fabio421

Bronto, its easy enough to remove the air pump belt for a quick test. See if the noise goes away. If you feel that you are going to get stranded you can leave that belt off and wire your A/C fan wwires together at the temp switch on the bottom drivers side of the radiator. If you want to eliminate the air pump you can run a 20" - 21" belt to turn the viscous fan off of the crank. Or you can do what I did and have some spacers machined so that the fan moves forward to the same groove as the power steering pump. This allows you to adjust the belt tension with the PS pump. Using the 20" - 21" belt doesn't allow for any adjustment. If you want some of these spacers, let me know, I can have more made.

bronto

My mechanic had the stethoscope on it for a while and says it's definitely the pump. Also says there's a little noise coming from the AC, but it's minor.

He also suggested the shorter belt off the crank in case I wanted to wait until I go on a trip to DC in early February to have the work done, but I'd just as soon get it done soon and not risk the pump seizing on me somewhere.

Oh well. I just sold him my old 525i in exchange for 928 labor, I guess that credit balance will be declining sooner than I thought