My poly-61 leaves sometimes notes ringing on.
Ringing isn't very loud, but I would be glad to stop it. Does anyboody know what to do? There isn't any connection between sounds or keys with the ringing. I suppose it's of some kind of electrical reason.

Well since that is an old piece , sometimes capacitors will leak and cause a high pitched noise . If the power supply is failing it could actually send to much current through . If the OPP AMPS start to fail , they will make noise also .