I'm sorry but I disagree with a lot of the posts from yesterday saying
that the problem is in the piano legs.

The piano legs may be loose, but that's not the cause of the problem,
it's an expression [a symptom] of the problem.

The source of the problem is the pump: the pump is not balanced.  This
can be several different problems; one could be that the posts and arms
are not properly lubricated so one of them is sticking and when it is
in motion it causes the pump to jerk and that jerk causes vibration
which you then see across the piano.

Another would be that one of the valves (unfortunately, usually an
interior valve) isn't sealing properly causing uneven vacuum through
the piano which you won't notice in the playing but you do notice when
you watch the motion as it is just slightly off balance.

Eli Shahar