pianoguycan

I have a Casio px330 and when hold down a couple of the keys i'I hear a soft popping/echo sound as the note is played, but If i go fast there is no popping sound. This is when I use modern or classical setting, but if i change it to strings, organ or any other settings there is no other residual sound coming from these 3 keys. I tried doing the reset procedure but I don't see any difference. Am I able to download new software instead or is it something else?