Well this is a new one for me. Experienced an occassionally but regular speedo clicking /clunking when driving along, perhaps every 15s or so, faster the faster you go. This was after alot of fiddling with the instument cluster to sort out a moving outside temperature gauge when you put the lights on (just needed a earth closer to the cluster).

As it turns out the trip reset is a bit sensitive and you do tend to knock it when tipping the cluster forward to remove it. This seems to knock one of the gears off its track or out of alignment, jamming the trip mechanism, the speedo stiffens up gradually until there is a big clunk and the gear jumps.

The cure: press the trip reset, and in future try to lift it up when tipping.