The magnetic ballasts have a hum or purr even when new, but it’s usually so slight you won’t really notice it.
But they do tend to get louder as they age. I have one that’s so loud it will drive you nuts after a few minutes. The hum, in my case gets picked up in the metal shroud which amplifies it. What I did, which helped, was to loosen the ballast and put a pump of silicone caulk under it and retighten after the caulk set. I also jammed a few wooden shim wedges between the shroud and the body of the fixture because it would start to vibrate like a big annoying tuning fork. It’s tolerable and I don’t use it that much, it’s still working, and I made that quick fix in 1990.
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