Hi everyone,
I recently had a very good deal on an ENGL Screamer 50 combo amp. At first it was perfect, but lately it's started making some sort of "crackling" sound. I'll describe it to you.
It's sorta similar to the kind of sound you get from a bad cable (in fact that was the first thing i checked), unplugging the guitar doesn't affect it (it keeps doing it even if nothing is plugged in), it comes and goes, sometimes starts right when i flip the standby switch, sometimes it takes a while before it starts, sometimes it goes away after a while, etc.. it's pretty much unpredictable really. Sometimes putting the amp on standby and then turning it back on makes it disappear, sometimes it doesn't.
One last thing, the crackle is very low volume, only noticeable if i'm not playing or playing at bedroom volume, and raising the master volume, gain, or any knob for that matter, doesn't amplify the crackle. In fact, it's no big deal since it basically can't be heard when i play live, but i'm still worried it maybe something that might get worse if I just ignore it.

Anyone has any idea what can cause this? Thanks