The loud and obnoxious "beep-beep-beep" of the little electric carts at ATL. Most other airports manage to run carts without the noise, so I don't see why it can't be done at ATL.

And a corollary to that, it is annoying that people should have to scurry out of the way to allow these things to pass. I have no problem with the use of the carts, but they are for moving people who can't walk well, not for racing from one gate to the next.