WIthout being there, I would say the AC compressor is locking up. Even though you are requesting heat, the AC compressor turns on to dehumidify the air coming into the vehicle. That way, the defroster works faster and better.
I don't know what type of warranty you have, but if you have a list of items covered, check for the AC compressor as a covered component.
Let me know what you find or if you have other questions.
Joe
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Sunday, November 15th, 2009 AT 10:30 PM