it's supposed to be alarming, so you can go turn the heat down once something starts boiling enough to boil over. Similar to a stove-top pressure cooker, you have to wait for the sound to know where to set the heat.

I use a glass milk watcher while boiling milk for making yogurt, and the clak (not clank) sound is not that stress inducing. On the contrary, it's a heartbeat which says that your milk is safe from boiling over and tasty yogurt prospects are still strong.

I think it's a matter of getting used to it.

Stress-inducing that's the right word to describe it