My 87 does the same thing. I had to take it in for the chirp and a knocking sound that was apparently caused by an idler pulley going out and was told that because that pulley was loose the belt lost tension and got dirt or dust underneath it.
I replaced the idler pulley and they wiped down the belt but the chirping is still there and it drives me nuts for the first 20 minutes of commuting.
Did you buy a new belt and did that get rid of the chirp?