There is a folk phenomenon, called The Hum

https://www.bbc.co.uk/news/magazine-13752688

https://www.reddit.com/r/todayilearned/comments/xlsdk5/til_t...

so my guess is that LLMs see The Hum in their training data, and then put the word "hum" in their output. Since humans occupy varied, small media bubbles, many haven't encountered text talking about The Hum at all. The LLM's use of the word "hum" then stands out as excessive and a tell. And a mysterious one!