Failing idler/tension pulleys can cause many issues with these cars including overheating, serpentine belt destruction, no A/C, no charging, etc. Replacing the pulleys in the car every 60,000 miles will insure these items do not happen. The pulleys are plastic and have a tendency to wear out and make noise.
Tensioners (Pulley #1 and #2)
There are two types of tensioners; hydraulic and mechanical. Everyone has different opinions on which one is better. The benefits of a hydraulic tensioner is you only have to change the pulley, while the mechanical tensioner requires the whole unit to be replaced. I have never trusted tensioners in any vehicle I own and just change the whole thing every time.
Idler Pulley (Pulley #3)
Notice the nub sticking out for proper mounting.
The idler pulley is there for belt routing and tension only. There is one bolt to remove it. The bolt holding the idler pulley in place also serves as 1 of the 2 bolts to mount the alternator.
All BMW E46s with the M54 share this part.