Why does my freewheel go 'snap, crackle, pop' when it gets wet ?
(It also misses on rare occasions, when wet)

How do I deal with this ?
Assuming it's water that does this, why ? It isn't rust because once it dries out it's fine.

It's not a Shimano, it's a chinese freewheel. The derailleur, which is the same brand ('Soleus'), looks like a copy of an old Japanese Sunrace derailleur.