Hello Bert,
The first place I would look is the MODULATION Wheel. Sometimes when taking off the cover or moving the keyboard this gets moved which then leads to some distortion of sound.
If it isn't that I would try PROGRAMME MENU, CONTROL, INITIAL and see if that cures it, if not do a 'Factory' initialisation but if you have some settings you wish to keep, make sure you save them to SD Card or Floppy before doing any initialisation.
Good luck! Audrey
[This message has been edited by Audrey Turner (edited 09-29-2007).]