Vintage Synth Repair

I just purchased very nice vintage E piano Yamaha CP10. It’s without problem, visually like a new one, everything works, there’s just one small thing:

in higher octaves there’s relatively strong “thump” moise on each key press.

I don’t call it problem, because maybe it was like this since the leaving the factory, maybe they did it intentionally… But probably not, it’s too loud IMHO.

I have also CP35 which hasn’t it.

Can somebody confirm how it should be?

Or what could be the reason, where I should start to find the aged or even defect (?) components? I suppose something in the key assign circuit, or triggering circuit, envelope (Sustain) circuit…? Could it be some capacitor?

Unfortunately I haven’t Service manual, it’s not freely accessible on the net, and I’m afraid that one for CP11 is different.

Thank for any hint.

Daniel Forro