Humming noise when braking

[highland512]

It's a low humming sound that starts when I apply the brakes somewhat aggresivly when coming to a stop (30mph-5mph) When doing normal braking or cramming on the brakes there is no noise. There is no vibration in the steering wheel or floor board. I just replaced the pads, roters, and one caliper last weekend with no fix, although it did help. 2004 F-350 with 107,xxxmiles. Any ideas of what it could be?