RV50 Buzzing Issue

[philiprst]

Here's a strange one.

I have RV50 that buzzes through the speaker even went the gain and volume are turned completely down. Happens on both channels. The buzz sounds like the mechanical noice from the transformer only ammplified. The only thing that makes it change is turning up the reverb knob. It gets slightly quieter at high reverb settings (with everything else off)

Any ideas?