The chain was replaced shortly before flipping the sprocket, .... I flipped the sprocket and the noise vanished.
Last year I put on a different chain on my Rohloff bike, not new but it was much less worn. Had noise and I had an odd feeling in the pedals, like the chain was catching on each link. And that odd feeling was stronger the harder I pedaled.
Flipped sprocket to a new side and everything was fine.
The teeth wear to the shape of the chain links, elongated links on a worn chain will wear the teeth differently than a new chain with non-elongated links. So, I suppose this should have been expected.
That is why some people on derailleur bikes will frequently change two or three chains on a bike in a planned rotation, in the hopes that they can wear out two or three chains for each cassette.