brando

I just bought a 5 spd. '97 Impreza Brighton wagon. There's a noticeable hum in the cabin coming from the rear of the car, faster=louder. It kind of oscillates too, so I'm pretty sure there's a problem. - Is noise from the rear common? - Can I assume it's the rear diff? (I'm new to AWD) - Is it horrible to drive with this condition, or can I get away with it for a few km? (I probably can't afford a replacement right away.) Any help greatly appreciated! --bh PS. Excellent forum!