Something is moving. Hard to hear it from here. :-)
If it is only the hot water, then it likely is caused by the expansion
of the pipes. In any case you will need to find the specific location(s)
that is(are) making the sounds. The solution may depend on the situation,
but often just a simple change in the way the pipes are supported will take
care of it. It could mean re-routing some pipes.
If this started right after the work, I would suspect a connection and
ask whoever put in the system to come out and take a look. They may just
quickly take care of it for you. They may have moved something or put one of
their pipes against an existing pipe etc. to cause the sounds.
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