hey guys,

got an early nineties fender twin amp and it's got this annoying crackle. It happens with nothing plugged in, no knobs are being turned. checked the tubes and isolated each one with new, working tubes. Had it checked by a tech and he couldn't fix it. He checked the solder joints and everything alright.
Any ideas of what it might be? ANy help would be appreciated.
PS: I've got a sound clip of the crackle if it helps anyone