Its the black alternator belt being ground down and slipping.
Sometimes I get a complete dusting of the front of the engine..

A couple of causes I have seen on my 25 year old 1GM:

1. Rust on the pulley surfaces grinds away the belt. A bit of work with emery cloth on the pulleys seemed to help.

2. The pulleys arent lined up, so the belt is constantly sawing on one side of the v-groove - I found about 1mm misalignment between the engine and the alternator, using a straight edge and a lot of fiddling. Adding a washer between the alternator and the engine block on each mounting bolt seems to help.