It's been happening for a couple days now and nothing has really changed in my setup so I'm wondering what steps I should take to debug the issue. It sounds like a needle on the end of a record. It's not consistent. Sort of happens and then disappears briefly but I would say it's crackling more than it's not. Don't know if it matters but it's a DB751 Head and a DB112 Cab. Things I tried: Isolated it was the amp and not cables/bass/pedalboard. It makes the sound with nothing in the input. Not interference from my phone (which normally can be heard through my studio monitors anyway). Raising the master volume does not make it louder. And the sound is present with the master fully off. Raising the gain does not make it louder. Replacing preamp tubes doesn't do anything Unaffected by the Effects loop. I tried bypassing it, putting it in series, in parallel, and raising/lowering the send/return volumes. I have another cabinet at my practice space I'm going to test tonight but short of that, anything else to try?