Vintage Synth Repair

Hello fellow synth enthusiasts!

I've spent (literally) years restoring my Univox Minikorg 2 and I'm down to one last lingering 'issue'..

There is often (30-40% of the time) an audible click (or snap) when
depressing and/or releasing a key. The click sounds the same whether the filters are totally closed or open, so it must be happening after that stage (and probably in the VCA/Envelope?).

Here are the easy-to-read schematics:
http://www.cykong.com/Synths/Korg%20700S/Images/Mini-Korg-700S-Schematics-Coloured.gif

I really hope someone has some idea!?

Thank you very much!
- Robert