I've had the same problems with my Z3 with the pulley squeaking soon after installing them. The problem is a combination of poor bearing used by Fennel Drive and the pulley spinning a bit too fast... IIRC given the ratio about 5500RPM the pulley spins above the limit for the bearing (15K RPM): the room constraint I guess didn't allow Downing to go with a slightly bigger idler (like the one BMW used for the tensioner). Fennel Drive used some ****ty a$$ bearings: the seal fails quickly and allow dirt to enter the bearing and damages the race. Eventually it will seize...
The bearing in the idler is a standard size (6203-2RSR) and the idler can be easily disassembled. Unfortunately as you push out the bearing the housing deforms and the new bearing won't sit tight anymore.
At one point I've almost had a local machine shop manufacturing a idler out of a block of aluminum to make it serviceable with quality SKF or FAG bearing and be able to go a tad bigger on the idler to slow it down.
I still have the draws of the idler if anyone want to machine them out:
http://www.mediafire.com/?y5nnwxum2jn