[schiffern]

That was what I thought at first too. However if it were pressure change, the sound would change when you move the knob, not when the hot water reaches the shower.

[zbrozek]

Also it's more-or-less the same pressure. It's the cold supply pressure pushing water out of your tank or through your tankless water heater.

[nemosaltat]

Yes, the thermostatic mixing valve is worth understanding- it’s pretty neat.