My 08 impreza has a loud humm

Hello, noises are impossible to detect from this forum. You need to hear the noise first hand with an experienced ear to know what direction to go into. Fortunately, there are shops near you with technicians skilled in diagnostic testing, and you can consult them to give you some guidance. Often times, a shop and a technician will listen before charging to give you some information and advice on how to approach diagnostic testing -- I'd ask for some local assistance. Good luck!