Buzzing AC15 with Weird Overtone

[dollfaceMusic]

Hey guys, I've been having this issue with my AC15 CC1 that started a few months after purchsing it used from a private seller. The only mod he did to the amp was adding a greenback. I have since done the bright cap mod, but the issue was present before the mod.

Issue: After I power on the amp and turn on standby, I get an immediate hum that gets really loud, then softer and then a bit loader again and stays there. As well, when I play certain notes on the guitar, I hear an overtone (just a single note) in addition to the note being played...SUPER annoying...coupled with a crackle of sorts that dies out before the original note or the overtone.

I've replace ALL tubes, no change. Tapped on PCB & tubes and I hear and amplified tap through speaker. However, when I tap on the chassis, no amplified tap. No oozing caps or cold or bad soldering. All speaker wiring and general wiring are intact and solid. Appears to be original factory work, no other mods.

Does anyone have an idea of what the issue may be? I've scoured the internet for over a month looking for an answer and have not found one. Possibly bad cap(s)? It wouldn't be an output trans issue would it? 

Trying to save the $$ by doing the work myself.

Any help is appreciated!!!!