my car humms when I break only.

What is your question?

took my car to les schwab three times. my brakes, roters are good. not sure if the noise I am hearing is coming from left tire somewhere in the middle of the break. but only when I push on the breaks you hear this humming noise. maybe my rear differential? or abs? help I was told I shouldn't be afraid there is no danger of some day not being able to stop..??? any advice would be much appreaciated.