Ahhh the phantom (occasional) clonk/clunk.
Mine was (occasionally) clonking when I bought it.
Still clonked (occasionally) after I taped and tightened the tank mount.
Still clonked (occasionally) after I greased and adjusted the steering head bearings.
Still clonked (occasionally) after I fitted a fork brace.
Still clonked (occasionally) after I fitted a Hagon springs and oil kit.
Still clonks (occasionally) no matter what the preload adjustment.
It's got no worse or better over the last nine years and twenty five thousand miles
and appears to have no noticeable effect on the handling but nevertheless can be effing annoying.
Will be simultaneously very impressed and very peed off if you succeed where I have failed and find out
causes it.