What's a sound that's been following you around for years?

I'm an electrician, so I'm around hums and buzzes all day. But there's this one — a low C, about 60 Hz — that I first heard in a substation maybe ten years ago. Now I hear it everywhere. In the mains, in the street, in my own head sometimes. I'm starting to think it's not a coincidence. Anyone else got a noise that's become a companion?