import:

did you replace or rebuild the calipers when you replaced the rotor. It is always a good idea to at least check them for fault free operation.

What CAN happen and may have happened in your case due to the sudden "appearance" of this problem is that the calipers ground your rotors flat. Rotors are supposed to be a little rough normally. Bad calipers can grind them smooth and, when this happens, either the pressure on the caliper/rotor or the reverberation caused by 100% surface contact (I don't really know the exact reason) gives off a high pitched squeal.

This is most noticeable when driving at slow speeds, although it can sometimes be faintly heard if braking hard at high speeds.

I would suggest you get your calipers checked out, and have your rotors examined to see if they are smooth. If so, you can have them reground/reroughed to avoid the cost of new rotors... they aren't shot, just out of whack.

good luck,
rt