Crackling

[lowell]

I have an amp on the bench that has a crackling Johnson Noise type of problem. It's definitely post PI and I've tried new power tubes and bias is good. The crackling happens sometimes when the amp is turned on, and sometimes NOT. None of the resistors related to the power tubes look burnt as that was my first assumption. Maybe a bad screen or grid stopper resistor. I began poking around with my signal tracer (capacitor with leads). I grounded one end of the cap and poked around thinking maybe I'd short the cracking to ground and find the culprit. It doesn't get rid of anywhere on the tube sockets. However, if the crackling is happening and I touch the signal tracer to the PI coupling cap just for a second and remove it, the crackling is stopped. This is the side of the coupling cap that is connected to the bias supply and power tube grid stopper. It's really confusing me. Any thoughts?

Here is a sample of the noise.

[lowell]

Welp... after all that I realized the amp isn't sounding right. So I had thought the amp was working but also had noise. At least now I can troubleshoot the problem a bit better being that there's a REAL issue with it. The worse the problem usually the easier the fix. We'll see.