I noticed recently that when I turn on my shower, I can hear the moment that the water heats up. The hissing sound gets slightly louder and its pitch changes. The pressure doesn't noticeably change during this time (and I can't see why it would), so this is a bit of a puzzle. In fact, in many places I've lived I've had a sense of when the shower water got hot, but I never consciously noticed what the cue was until a few weeks ago.
Apologies if this turns out to be more of a plumbing question rather than a physics question, but without knowing the answer it's impossible to tell ahead of time.
Edit: The hissing sound I'm referring to appears to come from the shower head.
