My mechanic can’t find anything wrong with my Forester. They can hear the high pitch hum coming from the rear, but its so faint that they can’t pinpoint it. It could be a wheel bearing issue or bad differential. They have checked it out twice. It kind of sounds like the high pitch hum you hear when passing an 18 wheeler on the freeway. Its loudest at 40 mph. If I speed up or slow down it disappears. The noise is gone when I come to a complete stop. I have a bad right head gasget but I don’t the two are related. I’m taking the car in to replace the gasget and would like to have the noise problem fixed at the same time. The car has 168,000 miles on it.
There was a recall on some Foresters as a result of defective rear wheel bearings.
Your mechanic might have no clue about this recall, so I would suggest that you call or visit the Subaru dealership, give them your VIN, and have them check to see if yours is one of the recalled vehicles.
For reasons that I will never fathom, less than 50% of recalled vehicles are ever brought in for the free repairs being offered.
Incidentally, you are making a big mistake if you only replace the right-side head gasket. Have both gaskets replaced at the same time.