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Turn the sound up, *without* hitting any key. Listen carefully - is there a buzz, or static, in the speakers?
Turn the sound down to normal levels again.
I assume that you have tried changes to MIDI mode settings.
I would recommend trying a factory reset of the keyboard, just in case it might help, e.g. for settings or a locked-up condition of a processor
Is there any sign of breakage or damage to the keyboard? if not, you might want to do a power-Off "rest reset" for overnight, with the keyboard unplugged (and any batteries, if any, removed also), and see if that helps.