I had the same problem with my 93 GLT. I found that it wasn't the compressor but the serpentine belt slapping at an idle. Does your noise disappear when you raise the RPM in neutral? If so you might have the same condition I did. While your car is idling and making the noise, try pushing against the tensioner (top of your engine - you'll see it moving a bit with the belt) to see if this quiets it down.

Changing the serpentine belt will solve the problem only for a short while. I had my original dealer change the belt twice, each time the noise came back (second time he tried to tell me the first belt was defective.) I had quite an argument with this dealer who then wanted to change the tensioner.. (this after changing the Sbelt twice and the noise coming back in 4 weeks each time). So heated an arguement that I quit using him. Then finally at the recommendation of the new dealer after several months of unbearable noise, I had the tensioner assembly changed and Sbelt again($470 with parts & labour)and this solved the problem only for around 4 weeks and the the noise came back the same as always. I suspect the original tensioner was fine, and the new serpentine belt was the only reason that the noise stopped. I have had a couple of other mechanics look at the original tensioner and they are at a loss to explain why anyone would want to change it, as all the pulleys appear to have no slack or play in them. I'm still going to head back to the dealer to challenge him on this issue (I hate having work done that doesn't solve problems and worse I hate paying for someones elses mistakes!)

The final solution I found after snooping through the brickboard and someone replying about a different way to route the sepentine belt... So I called up my new dealer and asked if they were aware of this 'fix' that I had heard about on the Internet. That got the dealer to phone Volvo and sure enough (if I understand this right) there is a different way to route the serpentine belt with a 'different smaller-sized belt', thus providing more tension.

I had a new small belt installed (no charge for labour) and so far after 3 months it's still quiet and no slap. The dealer did try and caution me that the new belt would put additional tension on some pulleys, but another mechanic scoffed at that notion, as the tensioner is what sets the tension.

Anyway I hope this helps you out. This was for a 93 and if yours is newer it might have already been addressed and perhaps you do have a bushing or bearing problem, but it's sure worth a try.