The tensioner is a known issue w/ Subaru. Most dealers have them in stock. DIY in 20-60 min. depending on your ability and tools. Might as well do BOTH drivebelts while your at it. If its a more consistent howling (almost like a superchargers whine) it's probably your AC compressor. Not exactly something you want to do at home..unless you just happen to have a R134-A machine sitting in the corner of your garage collecting dust.