How big was the washer , Dime, Quarter?? How loud is the racket???
It could be trapped in the intake, between the carb and the cylinder. Pulling the carb and the intake tube that attaches between the carb and the cylinder, you should be able to see it.
If it made it into the cylinder, I would have thought that it would have destroyed the spark plug. Pull the spark plug and use a small bent wire Piece of cloths hangar) fell around inside the cylinder.
I have the Onan manuals on my site. You can download the Onanparts manual here:
While your at it, replace the plug and the air filter..