I see this thread is a bit old, and honestly, I've meant to ask about this exact same issue myself for some time. Both of my outside mirrors squeal/squeak horribly (also in the 115+ degrees of Phoenix!) and it is just getting worse (and like nails on a chalkboard). I didn't equate it with the heat - which makes sense, as the parts change size/shape a bit. My other issue with the mirrors - and this one I've had an ongoing but only occasional hassle with - is that often times, when I remote start the car, since the mirrors don't...um..."un-retract" (detract? Open? Whatever) until a door is open, and I'm assuming that this is how it's supposed to be (although, since the lights under the mirrors are called "puddle lights", they don't really do any good lighting any puddles while the mirrors are retracted and the light is shining on the doors, not the ground, and if they don't "un-retract" until I open the front door, I'd already be standing in a puddle if one was there - right? Yes, that's petty - yet true, eh?). So getting back, my other issue is, often (but not always) a mirror will "get stuck" as it's trying to "un-retract" (that's my new word, I like it!), it's usually the passenger mirror, very occasionally the driver mirror (never both at once), so it just stays retracted, but it is TRYING to open yet just "clack, clack, clack, clack" it will just barely move out/clack-pop-closed/repeat until I can get in the car and start it up (actual start, not the remote start), even then, I will sometimes have to hit the mirror retract switch to stop it, then push it again - and it opens just fine. As much as I love this car, I will say, it definitely has it's quirks - a lot of them! Such is ever-evolving technology.
Anyway, I'm not really looking for an answer with this, as my car needs to get in for regular maintenance and it's still under full warranty, so I'll have them look it over. Of course, this will be one of those "unable to duplicate" gremlins. I'm just SO happy that I purchased the ESP on the vehicle. I just wanted to share my issues with everyone else, I'll try and post what Ford's service guys do - if they'll tell me.