Mesa Studio Preamp - "dirty pot" sound

[serrels]

I just noticed my Studio Preamp making a subtle "noisy pot" sound. It's constant and comes through all the outputs. It's not affected by adjusting any pots or switches... except the Master volumes. I noticed that turning up the Output Level makes it slightly louder, but adjusting either the rhythm or lead Master volume makes it much louder. Also, it doesn't seem to start until the tubes warm up.

Any ideas?

[Sweetfinger]

Likely a noisy tube, or resistor. The fact that it is subtle and is only affected by the MV's means that it is somewhere right before the master.