It is interesting to note that the 6303Z bearing is also used on my idler pulley on my RX7. Since I'm redoing the whole car, I took a look at my idler pulley. Spinning the pulley makes a nice metallic sound? This sounds similar to the sounds you guys have experienced. Bought a new bearing for this idler and it is marked 6303BULL. This bearing superseded the orginal 6303Z. I have not been able to locate specs on this one but it might be another option.