I have been chasing down a noise coming from the passenger side of the engine bay in my 1997 850R (155000 miles). It only happened if the engine was cold or if it sat for quite a while (maybe 30-45 minutes minimum). I used a stethoscope and due to the extreme vibrations it was difficult to pin point. Removing the serpentine belt narrowed the search to those components since the noise was not present when the belt was off. I had just done the timing belt and water pump so I went after some possible suspects that were not to costly to replace, the tensioner and idler pulley. I checked the power steering and alternator as well, nothing. I was left with my A/C compressor. However, the clutch was engaging and there was no discernible issues with the barrings. I took the car to several friends who are "enthusiasts" of sorts. Last week one of them confidently stated "you have air in your a/c system and the system is cavitating. The compressor is shuttering until it can equalize the system." I then connected the dots that I had recharged my system about 3 months early. The system was checked for air and sure enough there was quite a bit of air in the lines. The air was removed and the next few cold starts were perfectly normal. Problem solved

 

Good job!