Please put a 15 amp breaker back in the circuit if the cable is 14 gauge. If it is not 12 gauge cable, you've created a fire hazard.

I doubt that changing the breaker from 15 to 20 amp would eliminate microwave humming. I suspect the original breaker might be defective, so do as ionized said, but use a new breaker. What else is on that circuit?

Get a new 15 amp breaker, correct brand to fit in your panel.

You could even try putting the old breaker back in. Sometimes humming was just that it wasn't mechanically secure.