Hi
First of all, I'm not talking about the hardware problem that all the NP-30 seem to have ; that problem has been fixed on my keyboard.
This is something else : I've noticed a really high frequency sound, like a ringing noise, on some of the keys while using the piano voices (grand piano 1, grand piano 2). It sounds like a reverberation effect that's gone a bit wrong, and you can hear it when holding down a key (not by playing staccato!). It doesn't happen on every key, on 1/5 of them I'd say. Because there is usually complete silence around me when I play, I hear it a lot, and of course now that I know it's there, that's all I can hear. Also, the fact that some keys don't have that noise tends to indicate it's a ... problem.
For example, the G#1 key sounds great, but I hear the noise on G1. Noise is also present on F3, but not E3. Noise is also there on F3 to C#4, F4 to A#4, etc ...

It's quite frustrating, and I was wondering where it could come from. Bad audio samples for the piano voices (would be surprising from yamaha?) ? Hardware default on my item ? I have superhuman hearing and hear frequencies unknown to man ?
I've never found anything about this on the internet, If anybody owned an NP-30 I'd love to hear feeback on this.

Thanks
Greg