If what you hear, on a single note, is a tiny "glitch" in the sound as it decays:
. . . that's "looping".
It's a side-effect of using a short sample of a decaying string, and re-playing it, just reducing the volume gradually. It is one of the banes of digital piano designers (and players). You can eliminate it (during design) by recording the _whole duration_ of a note, and playing it back when the key is pressed.
You'll get the same effect on many other DP's. Go through the "DPBSD" thread; most of the DP's have audible looping noted.
"Beats" between two notes (at an interval of third, usually) are normal. It's a "feature" of any instrument tuned to the standard tempered scale, with evenly-spaced semitones.
"equal temperament vs just intonation"
for an explanation. Or, if the PX-330 supports "just temperament" (it'll be a menu option), see if that changes the sound.
PS -- Once you hear these artifacts of the technology, you _keep hearing them_. It's an ugly world.<g>