I bought a used 2000 4x4 offroad package F150 a few months ago. I haven't driven it much. I notice when at slow speed or whenever I let off of the accelerator there is a pulsating type of low squeel that comes from the front that is very vague you can barely hear it. Can this be a bad rotor or brakes? It seems to change pitch when I press on the brakes. I can only hear it when its quite enough.

 

It might be that the front rotors are warped being that they are junk composites and the wear indicators on the pads are rubbing or brake dust can cause some squeaking.