Liverpool with odd problem

[GernBlanston]

Hi everyone,

I finished a Liverpool project about 7 months ago and all has been fine up until now. Basically what I'm getting is this:

As the amp warms up I get this very loud crackling noise that gets louder if I turn up the volume control. When the amp is fully cool it can take up to 5 minutes to develop the crackling noise. When already hot it's immediate.

An odd side effect that I feel must be related is a buzz / hum I get when I put the amp on standby. This buzz / hum slowly dissipates once the switch is flipped to standby but is back full force if I put it out of standby and then back again.

Here's what I've done so far:
1) replaced all tubes with known good ones.
2) checked all B+ voltages and bias voltage (all within range)

The voltages never fluctuate when the crackling noise comes in. One the scope I see no real ugliness until I hit the plates of V3.

The only other time I had a similar crackling sound was when I had my old Marshall plugged into a bad or dirty wall outlet at a gig once.

I'd sure appreciate any tips or sage advice for where to look more. Thanks in advance.

Steve