Just today I got a question from a new ham on the pronunciation of balun.

He has been around the scientific community a lot (physicists, etc.) but not
many RF types such as engineers or hams.

He claims that he rarely has ever hear the pronunciation "bal uhn", and I've
rarely heard "bayl uhn".

I suspect it may be a regional-accent sort of thing. Some people
probably pronounce it based on its word origin ("balanced/unbalanced",
hence "bal uhn") and some based on its appearance (like "basic" or
"basted" or even "bailin' wire").