MGard

Luckily, after many efforts I think I could individuate the problem. It is the metal grate of the left bottom speaker which, not properly fixed, moves a bit and produces with some particular sounds and tones a sympathetic reverberation made of acute hypertones. Thank you.

In my GP-400 it seems that, with Berlin and Hamburg pianos (not Wien), in the 5th octave some tones (e.g. F and F#), if played "forte", produce an odd, metallic reverberation, quite distinct from the near ones, like if too many high hypertones were audible . Could you experience something similar? Thank you.