Hi. This post is really old but I got the same problem with a couple of vintage units (v3 and v4). The ‘pop’ sound when activate the pedal was terrible, but the solution was very simple. First of all, I performed a cleanup of the units with contact cleaner and a brush but it didn’t help. Later I found out that not only the switch activation caused the sound but any strong touch or vibration on the units (like activation of a nearby pedal). I opened one of them and found out that the tip connection of the output jack was a bit loose, so I bent it towards the center to make the connection between the jack and the plug stronger. Voilá! The pop sound dissapeared! I hope this helps anyone. Let me know if does.
Greetings from Chile!