Whenever the hot water is used, these is loud, very annoying "ticking" sound
coming from the pipes inside the wall. It starts rapidly and slows down.
Since the house is still under 1-year builder's warranty, we had the
subcontractor who installed the plumbing system look at the problem. Their
response is "normal expansion and contraction noise of plastic pipes", and
there's nothing they can do about it.

Are they telling the truth? Is there any way to fix it? Thanks.