I have a 2003 Subaru Forester XT Luxury manual transmission with 100,000Km. The last 5000Km I have been increasingly aware of noise - hum rather than grinding or scraping or shadder - from rear end which is louder the faster one goes. Not related to engine revs, but to the wheels turning. Turning Rt or Lt doesn't seem to influence the noise, which is rather like road noise, but seems to be arising from higher than contact between tires and road, and seems to fill the rear compartment of the car. I think about the differential, but I have never had to replace 1 tire, having only replaced all 4 at one time. The diff oil appears to be quite clean, which doesn't suggest excess wear of the Diff.