Hey CanadianMark,Mine does this. Only noticed it after I had it in for a new cowl when the blower motor froze. It does it every time I put on max defrost.
I saw your other post: http://fiestast.net/threads/anyone-elses-clutch-feel-like-theres-air-in-the-system.2086/#post-34506
Please take a look at it. I requested some additional information to see how to assist.
Ford Service CA