It's not the tensioner, it's the tensioner pulley. Mine did that for a couple of weeks and then it disintegrated as I pulled into my driveway. Lucky for me because I lost power steering once it blew.
I had the old mechanical tensioner, but the dealer no longer sells these. I paid $250+ for the hydraulic assembly from the $tealer. The mechanical one is still sold with pulley at Autopartsway for $45. The hydraulic one from Bimmerspecialist is about $140 (tensioner + pulley set). The mechanical and hydraulic ones are interchangable, although I think the latter is supposedly more reliable.