Is There A Mouse in my Preamp or Power Amp?

It's probably the treadmill that the mouse runs on, in the poweramp. Try a shot of WD-40 on the bearing ;^) But seriously, I would try the new tubes first. Failing capacitors can also cause odd sounds. If you have another source component like a CDP or tuner with a volume control, you can bypass the preamp to see what happens. Good luck.

What Elgordo sez. Drop in new line stage tubes and see if the problem disappears. Don't replace the phono tubes unless you only noted the sound when the phono was playing. Also, you may only need to replace the most sensitive of the line stage tubes, so move in stages front to back, 2 (L&R) at a time. I would doubt failing capacitors, but its possible. I highly doubt it is your amp. If it were you would hear this without regard to what was happening with your pre amp. Easy way to rule out the power amp - just reverve the channels (physically - not with the pre-amp reverse switch) the sound you hear should also switch - if it doesn't it could be your amp.