it could be the metal arm on the pad hitting the rotor

did you try puting anti squeak on them ?

 

Ultimates don't normally squeak too bad. Try removing them, cleaning everything, and beveling the edges of the pads at a 45 degree angle. I've heard this can help dramatically for sqeaky pad compounds. The leading and trailing edges in particular- some pads come shaped that way to begin with.

 

I had a high-pitched squeal coming from my right rear for a couple weeks. When I had it looked at it turned out that the rear axles were dry and needed to be lubed. I was happy it wasn't a wheel bearing.