1990 bounder turning gene on and off i hear buzz sound

Probably mislabeled
Do them one at a time see which one turns off MW
But you said turning off that breaker stops the hum
So you need to track down what is on that breaker

I'm gonna guess the hum is from the power converter that chargers your batteries

Could be a loose transformer winding in the converter, no danger just annoying
Or bad batteries causing converter strain, that is problem
If they are bad the need to be replaced