loud buzz on bassman 50

i don't play bass very often, but i do use a fender bassman to power a leslie 147 rotary speaker for guitar and organ. anywho, when turning the standby switch to on, there is a ridiculously loud buzz that sounds to be at about 60 Hz(synth sawtooth wave kind of sound) and when flicking thje standby back off it quickly fades and has an apparent phaser effect as it dies. it does this with every cabinet when this problem arises, including the bassman cabinet. the problem sometimes goes away by smacking the top of the amp, or pushing up on the capacitor with an oven mit. does this mean i should replace the capacitor? i'm not familiar enough with the electronics of the amp to understand precisely whats going on, and i'm afraid if this problem continues it may result in speaker damage or even damage to the amp