1992 Ford Aerostar

Heater problem
1992 Ford Aerostar 6 cyl Two Wheel Drive Automatic 312000 miles

Whenever I turn on defrost I get this odd grinding sound coming from the front of the car. Now the sounds are not constant but is on and off. I would say like every 2 seconds I get a loud grinding sound. What is
odd is this doesnt happen when I just use the vents for heat it only happens when I turn it to mixed or defrost.

Any ideas would be greatly appreciated.

Thank You
November 28, 2009.

Defrost setting turns on the A/c compressor, pulley bearing may be no good. Remove the serp belt and spin the pulley by hand, is it smooth with no binding?

Nov 29, 2009.