2004 Subaru Forester High pitch noice from car

You hear the noise when the engine is off and the car is stopped? Can you tell what area it is coming from? If front, could it be the radiator bleeding off pressure?

My high pitch hum seems to be coming from the rear. Starts at around 40 mph after driving a couple of blocks and gets louder the faster I go. Does seem to come and go on the freeway but definitely comes back when I take my foot off the accelerator and coast. No sound while I'm stopped. Turning doesn't seem to affect it either.

Chances are there is a problem with a bearing in the differential. Have you checked for any excessive in and out play in the rear tires? Any play where the driveshaft attached to the differential?