Why do kettles go quiet before they boil?
Hmm, good one.
So, err, why? A first guess would be that the noise as heating up is dissolved gases being driven off – does that make sense? Dissolving solids in liquids can be easier as temp rises, but gases work the other way around? – and then the actual boiling is the creation of the steam. But there’s that little gap between driven off gases ending and steam production starting?
This is, obviously, a question where the answer has been worked out already. So, is this one of those where first guesses are missing some vital fact that thereby makes a mockery of them?