These things are extremely picky about tubes as well... Make sure you research tube changes... For your problem, you may want to try cones underneath the unit in both directions -- tubes selection combined with the chassis noise of this thing or other components may be amplified in a microphonic nature. I had a friend that went through this and found his 'fridge on the wall behind the audio gear was creating enough mechanical vibration to disturb his AI Mod 3. His fix was isolation (zoethecus stand & cones under the preamp) and different tubes that were less microphonic.
Noisy AI Mod 3A?
I've now come across a Mod 3 and two Mod 3As, including my own, that are susceptible to what appears to be ground hum in some systems (but quiet as a mouse in others). Moving the preamp and wires and cables around seems to work in reducing but not eliminating the hum. Lifting the ground on various parts of the system or replacing the tubes doesn't seem to help. Any solutions out there?