LOPT squawking at HALF line frequency

[Kalee20]

I've had poorly-clamped 400Hz transformers singing at half (or lower) frequency. They sound dreadful.

Can't think of a mechanism why! It's like a 2-stroke engine four-stroking - but in this case there is a decent explanation why (the 'missed' stroke does a bit of extra scavenging, so the next charge does actually go pop with the spark).

I suspect chaos theory might provide an explanation.