Mid 90’s Line 6 AXS2 212 amp ( squeal in one channel)

[Oldmactech]

Neighbor gave me this AXS2 212 modeling amp that was dead when I got it. Solid state amp.
Turns out by cleaning the pots it’s works great. This thing has 4 banks so you can switch between all sorts of amp models. Works great except when I switch to bank 4 I get a loud squeal.

I eyeballed the board and see nothing obviously physically damaged. I don’t have a schematic either.
From the symptom, best place to start troubleshooting?

thanks.

[glebert]

Have you cleaned ribbon cable connections? Done a factory reset? Are you using a floorboard or controlling from the front panel?

[Oldmactech]

Acts Same with floorboard or front panel.
Did a factory reset.
Have not (but good idea) cleaned anything other than the pots and switches.

[glebert]

I can't remember if there are some socketed memory chips on this, but I would also make sure any socketed chips are fully seated.