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If it only does this with HOT water I would say it is an expansion problem.
As HayZee said if the pipes runs in a vertical direction to a horizontal direction this may be the problem.
1st off what material is the drian pipe...cast iron / pvc / abs ???
What size is the drain????
I can see it being PVC in too tight holes going horizontal, and each vertical section being up against a stud....This would cause a bang as the horizontal section tried to expand, and the flex in the vertical sections rapping against the studs......
If this is the problem you would need to get pipe away from studs and or band the pipe tight to the studs to stop it from flexing...
I do not believe it is gas pockets or superheated water.
Gerry
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