Knocking noise

solarflash

Cycling into to work this morning I discovered that the bike was making a loud knocking noise every 10 or so pedal strokes (sounds a bit like a stone colliding with the frame). I took the bike into lbs and he put it up on the workstand, checking for play in the cranks etc. Couldn't find any real issues. The noise only seems to happen when actually riding the bike. Anyone got any ideas what could be causing this?

supersonic

Have a look on park Tools for a step by step guide to diagnosing noises.