Confusing hum problem

[Poparad]

So on my AxeFX II, when playing through input 1, I have no issues, but when plugged into input 2 (when I teach lessons - I'm in 2, student is in 1), over time I get a horrendous hum that builds up. Unplugging my guitar for a few minutes and plugging back in fixes it, until it builds up again. Same guitar, same cable into input 1 never causes this problem. I'm currently using the rear inputs for both. Thoughts? I've had this happen with multiple instruments. I was doing a jazz gig a while back and the upright bassist and I were both running through the Axe, with him in input 2, and the same thing happened and we had to pause a minute to let it calm down before we could play again.

 

[chris]

It builds up??

 

[Poparad]

Yeah, it'll be fine, then it'll intermittently crackle on and off, then it'll be constant, then it'll get louder. It happens, suddenly, too, in a quantized manner. Like it hits a certain threshold and then is there or gets louder.