HELP high pitch whistle from tube amp

I have a unison unico with lv 2 underwood mods. Theres a very high pitch whistle from the left channel that remains constant through the volume range and input selects. Could this be ground loop issues? Or defective tubes? Outside of this recent glitch this amp is just glorious! Any advice much appreciated.
First, try swapping all of the tubes from one side to the other. If the noise moves with them, then you will be able to identify the tube by trading them back, one at a time.

Also, try swapping inputs with left becoming right side. Again, this will tell you if it really is the amp. Next, try swapping outputs, which will tell you if the problem is indeed in the amp.

I see you have already tried swapping input selects. Also, remove all but one input and test each at a time.
I will try all of the inputs and the outputs before I pop the top...I'll keep you posted. Much Thanks...Neil