I have a 2003 Sub Forester. The rear end is making a loud "humming" like noise (especially between 25-55 mph). Thought it was a bad bearing on the rear left side but when we got it lifted up and started spinning the hub there is no noise and it turns smooth. Has anyone had issues with this and what could it be? Was also thinking a differential problem??? I have close to 100,000 miles on it.

Put it in the air in gear (be smart though, don't have it close enough to the ground to touch when it's rocked), then use a stethoscope. You may need to have someone give it some gas, to get it going fast enough to make the noise.

rear diffs do not fail very often unless they run out of gear oil.

it is a wheel bearing.