[Darryl Ramm]

I would try to exclude tube/amp internal microphonics (which can sound like a whole lot of different things).

Have you played this amp recently with a different preamp at similar high volume and similar location of the cab to the amp head? Can you test that again?

I don't know how hard it is to get access to the internals with this amp, but you want to try gently tapping the tubes while playing, like with the eraser end of a pencil. Be very careful not to touch the high-voltage wiring/PCB/components, the voltages in these amps will absolutely kill you. Try also just gently pressing the pencil end it on the tubes and seeing if has any affect. You can use the Eleven Rack reamp capability to make testing easier. Try removing each tube (with the amp off of course) and reseating them, to try to clean any surface crud off the contacts. As you do that inspect all the valve sockets carefully for corrosion or other gunk, vacuuming off any dust or using a spay contact cleaner may also help (don't spray that stuff near warm/hot tubes). If you find suspect valves replace them with same type. I'd use Mesa genuine tubes. For the 6L6 even though there are two stereo channels I think Mesa recommends swapping out all four 6L6 at a time, not just the two 6L6 on any affected channel.

Actually checking if this static noise occurs when reamping is a nice way to exclude microphonic/feedback effects with the guitar.

Does the static type noise change if the amp cooling fan speed is changed?

Darryl