Not sure if your problem is the same but it sounds similar to mine - http://www.xoutpost.com/bmw-sav-foru...my-pocket.html

Hopefully your problem is just the tensioner - if it's the guides and you have as many miles as I do, you may want to peruse the list on page 2 which will give you an idea on cost for parts at least. Of course, what you have to replace will also depend on what has already been done i.e. valley pan, water pump, etc.