I've just recently put some Tiagra square taper cranks on my fixed and have been getting some kinda scary sounds when skidding.
I hear sort of a 'clunk' from the drivetrain (not really a clunk, but the sound is hard to describe) when I'm skidding and the tire grips the road again. Not 'nuts on the stem' skids, but slowing down skids. Other than this the bike feels and rides fine.
I've started using the chainring that came on the cranks too; a 39t inner chainring. It looks a tad flimsy, but it's steel and feels quite solid. The chainring bolts are tight, cog and lockring are tight, and chain tension is fine. The chainline looks maybe a millimeter or so off, but good enough. All gear teeth are fine too.
Does anyone have any clue what the problem could be?