You're experiencing a microphonic tube (or tubes). Hopefully, it's isolated to one channel. You can often find the bad tube by gently tapping on each tube's glass bottle with a rubber pencil eraser. The bad tube will be obvious because it will display the microphonic effect louder than the other tubes.
At least this has been my experience.
What type of tubes does your preamp use?
At least this has been my experience.
What type of tubes does your preamp use?