Yamaha Clavinova CLP 575 Key Sound Problem

by Leigh
(Macedon Ranges, AU)

My 3 year old CLP 575 is developing a worsening problem.

The F note in the 2nd octave below middle C, and sometimes the surrounding notes has a wooden clunking sound at start of the sound and the ongoing sound is brighter than other notes.
This occurs mainly at moderate and above loudness, and is particularly obvious when played in a scale or arpeggio.
Does anyone else have a similar problem or know a possible cause?