I believe that in that loop the fuel goes to the top of the tank, but it also moves to the top-front and top-back, depending on acceleration of the plane during the loop, leaving the clunk dry during deceleration.

If your engine is too sensitive to bubbles, or tuned on the lean zone, it will show that is not happy.
The situation will worsen for a half full tank.
Try the loop with a full tank.

Does it do it after flying straight inverted for a while?
If so, the clunk may not be moving freely to the top of the tank.