Clunk, clunk, clunk...?

It's not everytime...sometimes it'll do it, then stop. Sometimes it doesn't do it. Sometimes, while its going clunk if I spin the freewheel backwards (pedal backwards) a couple of times it stops when pedaling resumes.

The cranks are tight. The reason I suspect the freewheel is there is some play in it.

Help? Where should I look? Can the play in the FW be tightened up? The hubs are campy...will any FW fit?