Turn down the screen control (one of the knobs on the flyback), and see if that fixes it. Might be a CRT fault (internal short). Post a picture of the screen in operation.
Can you describe at all where on the chassis the buzzing noise comes from? Approximate frequency of the buzz (60Hz, 15kHz, etc.)? How loud is it?
Also, exact model number please or picture of chassis if you don't know and can't identify.