Keep looking. Keep looking for clues. You'll eventually find it. If it only happens in the big or small chainrings, that's a clue. If it happens regardless of which chainring, that's a clue too. What's the position of the crank when you hear the "clunk"? I once had a soft "click" sound with every pedal stroke that drove me crazy. When I found it I felt really stupid. It was the crank striking the front derailleur cable with every stroke.