Scratchy Noise in VTA ST-120 Amplifier

Hello, I have a VTA ST-120 Amp with the stepped attenuator ( currently set to full volume ). I use a variac to keep voltage around 115, have the Bias under 0.5 volts, use a Weber WS-1t with thermistor for rectifier. I use the TDR . I have a SS preamp that I am feeding the amp with. Speakers are Human 81 s, a fairly low efficiency acoustic suspension design

Recently I have noticed, either before the music starts, or if I turn the volume off on the preamp, or turn the preamp off, a low ( low volume , not low pitch ). scratchy, crackly noise - kind of like an old potentiometer volume control being adjusted. This is fairly recent -- I have always used fairly low efficiency speakers, and the AMP has always been very silent , even with your ear right by the speaker.

The noise doesn't bother me when playing music -- it either goes away with signal, or signal just overwhelms it . I do not hear the noise playing music. I am not concerned with the noise per se, more concerned with it suggesting something is going to fail. If there is some maintenance that I need to do, or components that need replaced, I would rather do it before an expensive failure occurs. One clue may be that checking bias seemed to increase the volume of the noise as the voltmeter leads were applied to the test points.

Any suggestions or insights into potential sources of the noise would be greatly appreciated.

Thanks.

First, disconnect the pre-amp and attenuator completely and see if the noise is still present. Is it isolated to one channel or both? Try swapping tubes with new ones if you have them or between channels one at a time and see if it follows. Next suspect would be a leaky filter cap in the power supply. I don't think a bad 5AR4 would propagate the noise past the filter caps, but one never knows. I have had brand new Russian 7025's, you know, the low noise 12AX7's, give me this exact problem right out of the box. I've also had bad, brand new filter caps, even ones that would make a sizzling sound just before exploding due to an internal short.