Constant Humm / Buzz but Works OK

Question

After just a few hours of operation, we started to notice a faint humming/buzzing sound. It is a little louder when you open the door. Seems to be coming from the left side. All microwave functions seem to work AOK. Cooks fine too, but the high pitch is too much to bare. I tested the H.V. Diode and it checks out OK.Thanks!