dpb wrote
Richard Fangnail wrote

In my apt the tapping noise can come from my ceiling or one of the walls. When the tapping got bad a few weeks ago,
it got bad in both places at once. Why would that happen, if the problem is due to the pipes rubbing against the
holders?

Sometimes the noise is in one of the two places and sometimes it's in both.

A slight tapping noise is not imo likely to be rubbing per se but more
likely thermal contraction/expansion--very similar to the occasional
pop/creak heard in a house's siding when warms up in sun on a cold day.

Thats rubbing.

I had one house w/ a very long horizontal drain run from a sink --
in it, there was a sound that could be described as a tap that was
actually a drip internally until that drain line finally finished emptying.

Thats unlikely to be his problem.

Someone mentioned a pressure regulator. What is that exactly, and how might it cause the tapping problem?

It would be a pressure-reducing valve and is a spring-loaded valve that could be installed to reduce house pressure
from the
higher-pressure distribution system. Normally, if these get noisy
they make a more distinct sound than I would describe as a tap, but I
suppose if it were buried in a wall or a ceiling somewhere it could be muffled.

But wouldnt produce that one of two places effect.