It could also be the pads you used. Parts-store high-metallic pads are prone to squealing at low wheel speed, and a high performance carbon-metallic pad will often make a humming sound, for example.

If you've got plenty of pad material and you're not hearing the typical high-pitched screech that brakes tend to make, start with the stop-squeal stuff mentioned above. If that doesn't help, it's most likely the brake pad material. It just happens to make a tone while it's rubbing against the rotor.