What is the noise coming from the rear end of my Forester

Bad wheel bearings perhaps. If not that then the rear differential.

Highest probability is a (or both!) RWB. U-joints? Nope...but nice painting. Diff? Unlikely, but possible. It can be helpful to have someone sit midway in the rear seat and try to determine left or right side, but that's a notoriously ineffective discriminator, as the sound travels across the unibody. Good wrenches will use a stethoscope or long screwdriver to "hear" the spinning bearing while running on a lift. Be prepared to eat both sides if a mistake is made if both sound pretty loud.