I've got a similar sounding issue when reamping through S/PDIF. Only a few of my high gain rigs are affected. Input and output LEDs stay green throughout (no yellow/orange/red). Similarities:
- Green Scream on (in fact, all the DS stomp boxes seem to be affected)
- Reverb on
- Cracklings starts a few seconds after either Green Screem or Reverb are turned on and doesn't go away.
I was able to find a workaround/hack that kinda works for me:
- Disable reverb. Fixed the problem on most of the tested rigs.
- In one case when not even that worked, I re-enabled reverb, entered the reverb settings and dialed the "Mix" knob to "0% (dry)".
Feels a bit like debugging at work.