:Why does hot flowing hot water sound different than cold flowing
: water? When I start the water running in the shower before I undress,
: I can clearly hear when the hot water gets there.

The valve-heating-up explanation is likely part of the answer, but I have
heard the same phenomenon in partially open 1/4 turn ball valves which
don't close up significantly when temp increases.

Another possibility is related to how the noise is generated in the
first place: cavitation at the orifice. Under proper circumstances
when water pressure is released at the edges of a valve opening the
dynamics pull tiny transient near-vacuum bubbles which quickly
collapse with a shock. Add up the tiny "clicks" from these events
and you get the "hiss" of water escaping through a valve.

Note I said near-vacuum. The bubbles actually have a measure of
water vapor in them, according to the vapor pressure of water
at that temperature.

Now as water heats up, its vapor pressure increases. This changes
the absolute pressure of the cavitation bubbles, affecting their
size and energy, affecting the "clicks", affecting the integrated
sound.

A similar effect occurs with a metal teapot on the stove. As the
water heats up the sound changes as increasing vapor pressure of
the heating water changes the characteristics of the transient
bubbles on the kettle bottom which make the racket.

In a valve, the water is literally "boiling" in much the same
way.

Incidently, cavitation is violent enough on a microscopic level
to hammer tiny bits of the valve edges away, eventually
destroying the valve.

Bob (in an alternate life is a water plant operator) Miller