Last time I adjusted the chain I didn't do it properly... I just picked a spot and began adjusting to roughly 30-40mm.
I now found the tightest part of the chain and measured, it was 20mm :O
Adjusted it to 35mm, sound is gone
But there probably is a tight spot somewhere along the chain, and one o-ring on the master link has been damaged, so it's leaking red stuff from it :/
Probably due for a new chain anyway.