If it is under or near the air filter housing, then it is probably a vacuum leak. If you had the Contour serviced recently, the person might not have put the air filter and housing back properly. Other sources could be the area next to the fender where the bottom housing is supposed to join another oval (?) black plastic. In addition, there are two more holes at the bottom portion of the bottom housing. One (closest to windshield side) is supposed to have a foam filter. If the foam filter is dirty, clean it (or buy another one). The other hole (closest to radiator) at the bottom of the bottom housing sits on top of a black plastic tube (with a screen inside). Clean the screen (with some tweezers and a some q-tips) as it is probably full of leaves, twigs, dirt, etc. On reinstalling the housing, note that the bottom housing needs to sit complete on top of the hole (radiator side) with the screen and line up with the "chair" at the corner nearest the windshield side.
Also note that there is supposed to be a thick rubber band which holds the bottom housing to a hook by the fender. If that is missing, it could cause the housing to be loose (and so the humming from a vibration). Another item which could cause the vibration is the clips for the housing might not be installed properly.
There are other vacuum hoses near the inlet duct (accordian tube) but those will probably trigger a Check Engine Light if there is a vacuum leak.
If you do not have a owners' manual, download for free,
www.fleet.ford.com
