I have a '97 Grand Caravan, ~125,000 miles. When I turn on the windshield defrost, I get a noise from the enging compartment, like metal on metal. The noise is only there for defrost, and does not vary with fan speed. The noise does get louder and faster with increased engine speed. Other vent positions (head, foot, mixed) don't have any noise, and the temperature and A/C on or off selection does not affect teh noise. It only comes on in defrost. I'm at a loss to explain what changes in the engine when defrost is selected, and why it would change with engine RPM... Thanks for any help...

Turk