I had that happen on the D15b2 on my 88 DX...turned out to be the compressor pulley (sanden type). Unfortunately, at the time i was too poor (college days, too much boozin') to get it fixed, so i took the belt off. In south Texas, i rarely used the A/C anyway...

GOD NO A/C sux in Tx....

I hope that's not your prob...i would also check the idler pulley (if you have one), because some times when they go off kilter, it makes a similar noise.

Again, not an expert, but thats my experience.