Hi,  If the piano shakes, there is something out of balance and the
motor/pump system is most likely to be the culprit.

So the most common cause would be either the motor itself or the pump
is not balanced.  That is, the center of mass of the rotating part is
not at the same location as the mechanical center.  That's like having
the tires on your car out of balance.

But the other possibility is having the force through the system out
of balance.  Picture a bicycle going up hill with a little wobble in
one wheel so that it touches the brake pad every time it goes around.
It would be like someone grabbing the tire every time it rotates and
you could feel it as you pump the pedals.

So, you should take off the belt and see if the shake is still there
with just the motor running.  Probably not.  And the pump doesn't
rotate very fast so that's probably not a problem either.

Then try to turn the pump pulley by hand.  It should turn smoothly,
especially if the tracker bar is uncovered.  Now put a blank roll on or
tape over the holes in the tracker bar.  There should be a resistance
whenever one of the pump sections tries to inflate the reservoir (as
the associated bellows closes).  It could be that one of the inlet or
output valves is stuck (or leaking) so the system meets (or lacks)
resistance once per revolution because the back-force varies as the big
pulley turns.

Hope this helps if a loose leg doesn't solve the problem.

Regards,
Craig Smith