We have a loud knocking sound that is heard about 60% of the time in the wall of our main floor after the hot water has been running in the sink or dish washer. It doesn't happen in the cold months, just in the spring, summer and early fall. It is VERY loud (sounds like a hammer but just one bang). There will be one knock and then a few seconds later another. This carries on for about a minute or two after the water has been turned off.

Any suggestions?

Thank you very much.

If you have PVC plumbing in the house, it may be that the plumbing runs through framing members that tight (or the pipe is in a bind) around it. PVC expands and contracts with temperature changes, and if the pipe is bound tightly enough, it will "pop" or "thump" as it slides through the tight spot. In cold weather the framing members are dryer and shrink (due to low humidity) so the sound is not heard.