Discussion about the Korg PolySix synthesizer

Fired up my P6 today, as it has been too quiet for too long, but found 
that it had a strange kind of background noise. After opening the filter 
entirely, no resonance, bypassing the FX, setting the VCO to PW, pulse 
around 4, it is very obvious, that two keys pressed produce a sort of 
ghost note, that sounds like a multiple of the frequency difference 
between the two keys, like a sort of cross-modulation. If I press E6 and 
F#6, I can also hear an E3. The ghost note is also there using a saw 
wave, just not so prominent.

I don't think it's suppose to do this. What could be the reason for 
this, and how do I fix it?

Thanks

Frank