You might have a bad wiper control board.
GM had some problems with the windshield wiper control board (bad solder joints on the connector).
The bad connections tell the control board where the wipers are. It thinks the wipers are in the middle of a pass so it turns on the motor trying to bring the wipers to their resting position.
You can either remove the board and re-solder the connections (should be visible with a mag-glass) or get a new board.
A new board for my 97-99 GMC Sierra costs about $25.
I re-soldered mine and it solved it the problem.
Check with the dealer for a re-call and ask a mechanic there.
Thursday, June 14th, 2007 AT 10:15 AM