I recently purchased a NUX NPK-10 electric piano, as a step up from a Yamaha keyboard. I love the instrument, and I'm enjoying playing it. However I have noticed a bizarre effect that I cannot explain.
When I strike g5 and d5 simultaneously and hold them, a noticeable and almost obtrusive overtone sounds, an octave up from the g, and sustains as long as I hold the two notes. The effect is also noticeable, though less pronounced, with the fourths ranging from c5/f5 to e5/a5, and absent above and below that range. I don't get this effect on other intervals in that range.
It's not a discordant tone, but it's unexpected and a little distracting, sounds almost like something in the room is resonating with the piano, or within it. At first, I suspected sympathetic resonance from one of my other instruments, or else a loose screw near the piano's speaker.
However, I find that when I strike the same notes while holding down the damper pedal, I do not get the overtone, which suggests that this is not a physical resonance issue at all, and now I suspect that this is a software issue, specifically, this is how this instrument models the sound of those notes played together, in "grand piano" mode, without the damper.
Can anyone help me understand what's going on here? For example, is this a known issue with this particular keyboard? Do other electric pianos have similar issues? Is there any way to correct this? If this is the only problem with the instrument, I can deal with it, but it would be nice to not have that bonus note :)