Clank from freehub?

Incidentally, this happened to me too recently.

Was the clanking accompanied by a slight skipping? As if the chain and cassette weren't mating properly?

That's what I thought initially, and blamed it on the KMC quick link I had installed on my Shimano 105 chain.

Eventually I discovered if was gunk in the freehub socket.

The reason for the clanking was the pawls were not engaging fully, due to too much gunk in the freehub socket in the wheel (Easton EA50 2008)

I took a couple of pictures (before and after cleaning).
This was after 18 months use and regular cleaning, which I obviously need to revisit...