If I remember my O level physics correctly, it's due to bubbles of steam forming and collapsing that makes the noise. They don't have sufficient internal pressure to support themselves against the water pressure.
Living in a hard water ares, we notice it more when the kettle has just been descaled. There are fewer rough surfaces for the bubbles to form on so they are larger and collapse with more of a thump. As you say, the noise can drown out a conversation.
'Cos the bubbles are now hot enough & stable enough to get to the surface & escape without collapsing?
Anyhoo, it's better that the kettle sings the way it does. I've seen several reports of water heated in a microwave becoming superheated - i.e. no local bubbling because of sharp edges & local impurities. Then when disturbed, by, say taking the cup out of the m-wave the whole lot boils over catastrophically, almost explosively, causing bad scalding.
Location: The Burrow, N53:48:02 W1:48:57, The Tin Tent - EGBS, EGBO
It's always a good idea to let water, or any other liquid, heated in the microwave stand for a short while before removing it or adding anything to it. Careful experimentation has taught me that 2mins45secs on full power is sufficient to heat a cup of milk if I fancy a milky coffee. That applies ONLY to my microwave.
On holiday in Tenerife, the hotel room had a electric plate hob cooker. Put just enough water for cuppa in kettle and placed on hob. Minute or so later heard a tapping noise slowly getting more frequent. Looked around to the cooker to find the kettle rocking from side to side with increasing frequency. Eventually turned into a frenzy like something possessed.