I believe that you are experiencing a common problem with the Contours known as "Moosing". This humming noise from the air filter box usually occurs only at idle, then disappears after you race the engine (if stopped) or drive away.
There are many posts in this forum about the problem. The fix is to put a 1/2" plug in a 3/4" hose with a small hole drilled in the center of the 1/2" plug. The plug with the small hole reduces the amount of air to the idle air control valve and stops the humming.
Again, search the forum for the main moosing fix (has great pictures).
