Driving my car last weekend I noticed a bearing noise while low speed / low revs - it was intermittent but I could replicate by running the car a couple gears too high, the car at about 700rpm < not something I would usually advocate. It certainly wasn't bad, but I'm in the habit of getting things sorted when they occur.
I took the car in to the local indi and they had a look and informed me it was the belt tensioner. The unit fitted to early Z4s is mechanical and isn't very well designed
, BMW recognised this and 6 years ago replaced this with a hydraulic unit - hence pre 2005ish in the title. It is a relatively simple job, apparently, with 1.5 hours in labour. The parts cost about £270 inc vat - that's with a new belt. The tensioner itself is about £190 inc vat, but there's a list of other bits to go with. Part replaced I have not been able to replicate the noise 
May car is a 2003 and has done a little over 70k miles. I don't know how urgent a job this would have been - it was the spring in the mechanical tensioner that was making the noise. Not sure if this could be a DIY job, but I don't have the confidence to start fiddling with modern engines... especially one seeing occassional track time
If you have a later car with the hydraulic unit you shouldn't have any issues.