The bearing in the pulley is spinning all the time, so if the noise is only on start up for a few minutes, when the pump is engaged the noise is most likely in the pump not the pulley bearning. There is a check valve to keep exhaust gases from entering the pump, if it fails the pump will seize-up with rust. If that has happened the pump and check valve must be replaced. It also is normal for the pumps to make some noise when engaged, it is moving air both being drawn into and out of the pump. But should not be a belt noise. My name is Chris Brown, I own C & M Hydraulics we have been is bus. since 1994 and all we do is remanufacture MBZ power steering gearboxes, p/s pumps and air (smog) pumps with electro-magnetic clutches. We deal with only larger parts dist. however most online dist. can get our parts if you ask. if you have any questions feel free to call. 702.734.8090.

Chris