HI! Welcome to FTE.
Sorry I am slow in responding. I have been out of town a lot the last week or so.
I can not say for certain, but I seem to remember this issue from several years back. And I do think it was the blower motor.
I'll search a little and see if I can find anythign.
You might ask in in the Heating and A/C forum to see if any of those guys knows for sure.
http://www.ford-trucks.com/forums/forum61/
Again, welcome to FTE.