Quite a few years ago we restored a Duo-Art with a steamboat pump and
it also had a "rumble".  The problem with ours (after much trial and
error) was that the springs on the large pumper pneumatics were not
strong enough.  After we replaced the springs the "rumble" was gone.
It is worth a try.

Let us know if this solves your problem.

Bill & Dee Kavouras