[wiz043]

I get a screeching sound from mine occasionally. I think it happens when the brakes were wet and maybe some light rust developed that is getting scrapped off?

That will certainly cause that type of noise but post #3 appears to be a different issue.

 

[wiz043]

It is usually worse in wet conditions but only when reversing and applying the brakes. No nosie when braking otherwise.

Has it always done this and does it only do it in your first drive of the day.?
It can possibly make this noise initially if condensation formed on the rotors over time or they were previously wet from your previous drive.
It can occur particularly when reversing and lightly touching the brakes.
Next time before you drive the car, check the brake rotors to see if they appear brown or 'rusty' as the squeal/squeak you hear may be the brake pads removing the 'surface rust' when applied.
I live in a warm climate but I will experience this noise momentarily if I wash the car and leave it in the garage overnight and reverse out of the driveway the next day.