Electric Motor sound cycling on and off when engine idling.

I noticed what I thought was the fuel pump cycling off and on when I was standing at a traffic light yesterday. I emailed a mechanic friend who said that the electric fuel pump runs all the time and it would not cycle on and off. He thought it might be some kind of air pump for pollution control.
I checked the AC and it didn't seem to be running either.
I don't remember hearing this sound before and am a little worried something is about to fail. Car seems to be running fine. Any ideas?