The reason BMW changed the mechanical tensioner to hydraulic tensioner on the M54 engine is due to warping of the OFH. The mechanical tensioner has 2 points of attachment, the hydraulic tensioner has 3 points of attachment with a stronger bracket. When the OFH starts to warp (due to the only 2 points of tightening), there will be noise coming from the front of the engine, very hard to pinpoint. Might sound like a failing WP bearing. BMW issued a service bulletin (SIB 11 04 03), which has the subject as "Noise complaints from mechanical alternator belt tensioners".
This thread created by Eurodavid has the issue with the fix on Page 3 - it's post #46. In detail.