That's really cool. I'd never heard of it before. First thing I did was check online that it's not some youtube hoax.

Incidentally, if you try this at home be prepared for "Whatever you're doing, stop it. It's really annoying" even when you eventually get it to work.

Works with most hot liquids with powders, I think I first noticed it in a mug of instant hot cider......

https://en.m.wikipedia.org/wiki/Hot_chocolate_effect

The hot chocolate effect, also known as the allassonic effect, is a phenomenon of wave mechanics first documented in 1982 by Frank Crawford, where the pitch heard from tapping a cup of hot liquid rises after the addition of a soluble powder. It was first observed in the making of hot chocolate or instant coffee, but also occurs in other situations such as adding salt to supersaturated hot water or cold beer. Recent research has found many more substances which create the effect, even in initially non-supersaturated liquids.
It can be observed by pouring hot milk into a mug, stirring in chocolate powder, and tapping the bottom of the mug with a spoon while the milk is still in motion. The pitch of the taps will increase progressively with no relation to the speed or force of tapping. Subsequent stirring of the same solution (without adding more chocolate powder) will gradually decrease the pitch again, followed by another increase. This process can be repeated a number of times, until equilibrium has been reached. Upon initial stirring, entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases

Fascinating.

Now I'm wondering if the powder has anything at all to do with it; or it's just the entrained air bubbles from the stirring being released and disappearing...although your statement about "equilibrium" seems to indicate the powder (until full dissolved) holds onto the air bubbles/supplies a surface for nucleation (I think I used that correctly).

So: would talcum powder work? hmmm

Aww I was hoping he would stir it again. I've done this since I was a kid. It's just hot liquids. If you stop and stir again, it starts out low in pitch again.