Brum noise from a Mark Levinson No.532

Hi! I wonder if some one have a god guess about this one.....

When POWER is pressed the amp sounds like BRRRrrrrrrrr.........
Stand by does not change it.

VERY audible for half a second and then barely audible. Never disappears....
It never affect the sound to the speakers. They are dead quiet.
My guess is that it could be some mechanical vibration but where to start looking….?

Maybe I’m wrong….anybody?