Microphonics seem to go with certain tube (DHT) designs. Turns out that I generally love the sound of such tubes. They say isolation is the key, but I have found that to be quite a challenge when addressing the problem. In some cases, the answer turned out to be part of the problem! (I wonder if those copper tube sleeves might not be helping).
I did not get a good look at the circuit board, but it likely has those snap in connections that I hate so much. They can seem to be solid with problems not seen. Maybe you can use De-oxit on those connections to see if that has an effect on the problem. Switching back and forth, can in rare cases, have the just vibration from the mechanical switch action to temporarily remake a connection. I doubt that it is the switch itself, but something out there hanging by a thread (electronically) that is making and breaking contact.
BTW, the switch itself is likely sealed and cannot be cleaned with De-oxit. That is also to say that oils and other contaminants won't be seeping in over time.