Humming Noise when car is on.
 

What is that humming noise that I hear coming from the engine compartment when all the windows are closed. I don't remember hearing it before. If I open the window I can't hear it. I looked under the hood, but didn't find anything wrong. Can you guys check your cars to see if you got the same thing? It's not a loud noise. It happens all the time. No matter if you’re idle or driving. I always have my radio on, but lately I have kept my radio off. Maybe it's been there all the time. Let me know guys. Thanks. P.S. it sounds like a vacuum leak sound.