Disklavier

For lack of a better explanation - when I depress the sustain pedal and hit
a key from near left of middle C up I hear a distinct thumping sound after
the sound of the note.  The technician for my area has never worked on my
type of piano so he doesn't know if that's the way it should sound.  I don't
think it is because it's very annoying.  I have talked to Mark Wisner at
Yamaha and since he can't hear it over the phone he's not sure of what I'm
trying to describe.  Has anyone ever heard of this, know if it's a problem,
or should it sound like that?  Thanks, Rick  (rickconti@...)