It's funny how your brain blocks out certain things - sights, smells, or in this case, sounds.
The stage seemed fairly quiet - until midmorning, when the gaffer got on the walkie with a question:
"The sound guy's going nuts. Can anyone find the source of that noise?"
"Uh, what noise?"
"That humming noise. You can't hear that? Wait until we roll again."
Sure enough, as soon as we were made aware of it, we all heard it - and boy, was it loud*.
Usually, humming noises on sets are, in fact, our fault. When certain types of lights are dimmed down, they hum (skypans are notorious for 'singing'), but we didn't have any of those lights burning at the time.
After all of us had made repeated circles of the set during takes (while tipping our heads from side to side trying to figure out where the damn noise was coming from, and getting scowls from the PA's for walking during takes), one of the guys said "It sounds like it's coming from under the floor".
Turns out, this stage has it's electrical transformers in the basement (good idea in theory, bad one in practice), and we could hear them humming through the flooring of the stage.
Transformers are usually placed outside the building. Seeing (or hearing, as the case may be) them under the floor is a first for me.
*Loud is a relative term. Sometimes I'm really surprised at what the sound department can hear - whispered conversations from three rooms away, heavy breathing, weight shifting from one leg to the other. I once worked with a sound recordist who was notorious for screaming "No swallowing during takes!" and "Stop breathing through your noses, dammit!".
This particular noise - although we could only hear it when the stage was quiet during takes - was loud in the respect that none of us were surprised that the sound guy could hear it.