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You could try the factory reset for this model - which is, to power On the device with the Select knob pressed in, until 'FACTORY RESET' shows on the screen.
Some of these units - a specific keyboard batch at a specific factory - had factory-recall type defects. Maybe you are only now hearing it and aware of it owing to some fear of the kids having damaged it and listening more carefully as a result?, but in any case, contacting Yamaha and asking if this might be a factory-produced defect and whether it might be covered by warranty repair terms, is a good idea in this sort of case.
Did you purchase the NP-30 in Australia, or from some other country? this determines what national branch of Yamaha you could contact about the matter.
Also you will need to know where nearest Yamaha-authorized repairer of this line of instruments is located. Please tell me your region - as indicated by post code or postal code, or by town/city and province/region. Thanks.
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I regret that my information source seems to have been wrong about that reset procedure.