Brandubbs

Hiya, I own a Casio WK-3200. Am a second owner. I am also selling it as I am looking to upgrade so I want to know if I can fix it. The person I bought the keyboard from wasnt the owner, but her sister owned it and was selling for her. She said her sister had barely used it at all. When I got it, the keyboard looked great, well kept so I dont want to blame damage from the previous owner. Problem: So all keys work fine except for C2 and C3. The issue is when these two are pressed with a bit more force, either individually or together, there is a rattling sound or like a distortion. When I play a C octave as bass notes on the left hand and a C chord on the right hand (with the two keys being pressed mentioned above) it is highly likely that this distorted or like rattling sound occurs. It makes playing the notes sound bad, happens in both speakers. Touch sensitivity is set to 2, which is good for how I play but its just the two C notes ruining it. If I set the tranpose to '01' instead of '00' it fixes it but it changes how everything sounds and I am not a fan. This ringing/rattling/distorted noise happens on any touch sensitivity just at different points when pressing the keys. Its not a matter of 'just press the keys with less force', I am selling this keyboard and it shouldn't happen so I am asking for a fix for it so its as good as new to go to a new owner.