I had the same issue with my Privia PX-860. In my case the problem was being caused because the piano was placed almost touching the wall of my bedroom. This may be cause due to resonance or something like that. If I move my piano far from the wall, like one meter or so, the problem is solved. Does anybody have a define solution for that? My bedroom is small; therefore, I cannot place the piano far from the wall.
Thanks a lot!