Start by removing all objects from the piano.

Make sure the lamp that is used for sheet music reading doesn't have a vibrating filament or something else in it that is vibrating. Make sure something near the piano, a picture, glass ornaments, curio cabinets etc., floor vents etc., are not the cause.

Check to make sure that something, a pencil, paper clip or something similar hasn't accidentally been dropped or pushed onto the sounding board.

All of these are educated guesses from here of course but, everything mentioned above, screw tightening in particular, and key locks as well should be checked.

Kawai is very good about taking care of any problems. If the dealer isn't willing to help then, by all means, contact Kawai directly. You'll be surprised at how quickly they will respond to this.

Tilex huh? Interesting. Very interesting.