Sympathetic/ Resonant frequency with a C+?

[fdesalvo]

Failing filter caps can cause this phenom. Have you changed speakers recently?

 

[fdesalvo]

I'd check your invoice if it's still around to be certain what was done. They might have just given it a checkup and cleaned the pots and such. I would suspect your filter caps are going.

 

[Boogiebabies]

I have a Marshall cabinet that is sympathetic to an A power chord in my rig room.
Anywhere else in the house it's gone.