Joined: Feb 2013
Posts: 1,472
Likes: 11
From: Near St. Louis, Missouri
Bikes: Giant Defy Advanced, Breezer Doppler Team, Schwinn Twinn Tandem, Windsor Tourist, 1954 JC Higgens
If it was a chain with a stiff link, it would be more like every 3 revolutions. Also the clunk would happen at different times depending on which cog you were on in the front. Even if there were evenly spaced stiff links, the clunk would be different depending on the gearing.
If the BB is OK, then I'd look at the pedals, crank arms, and seat.