I joined this group after finding your excellent, extensive posting of your build. Terrific detail and great pictures. My after retirement gig is rebuilding all Mazda 5 speed RWD transmissions including turbo. Current projects are 79' RX7, 2-Spec 90s Miata rebuilds, and S5 turbo. I'd be happy to help sort out your trans noise issue. With a new rebuild noise issues tend to be gear or clearance related and not immediate bearing failure. Did you purchase an aftermarket bearing kit for your rebuild and do you remember what bearing you installed in the center C/S position? The picture of your assembled center bearing housing isn't clear enough to determine what bearing was installed. However, from the picture it doesn't appear to be the bearing that I would expect for a Miata gear set.