I've used freesound.org in the past for some good ambient background audio - I recommend giving that a try.

I KNOW I KNOW! (oh oh oh)

Some certain lights make this special buzzing sound, and I think that might be really good.

Another suggestion I'd throw out there is the muffled sound of cars driving somewhere outside.  Or perhaps a muffled plane flying overhead.  I can hear that right now which is a annoying because I'm trying to record.

In an office, a computer might make a humming and sort of scratching noise.  In TF2 there's a sort of noises like this in several of the intel rooms.

Perhaps you could try to find the sound of old cars driving with old horns going off.
Apply a lowpass filter in Audacity and it should muffle it to sound kinda like it's coming from outside.

I was thinking of a modern light buzz, but I feel like old bulbs might have made a buzz as well, though a special buzz and less loud.

You could do like in Call of Farquanglu and have their chairs creek every time they move.  I really liked the editing of sound in that film.

Also, you might add some music.