What is the humming noise coming from rear of car?

What is your question?

The humming noise starts either when outside temperature is warm or hot or when the car warms up and the sound comes and goes while driving. Noticed when braking the sound starts or stops. The humming is high pitched. Just recently had car checked and they thought it was caused by a hole in the muffler and had it replaced but the humming is still there. This seems to be very hard to diagnose. Hope someone has the answer.