I have a Mesa Boogie Dual Rectifier. Whenever I use channel 3 (Distortion) I get a loud buzz. The other 2 channels are ok. Why would this be?
It sounds like a preamp tube issue. Just swap out the preamp tube which corresponds to channel 3 and you should be set.
My guess is possibly a bad preamp tube, one that only affects channel 3. Just a guess.