Ben,
Excessive vibration...might even be your crankshaft. Now that I have my recently rebuilt MB5 more broken in and I'm getting closer to red line I'm definately noticing more vibration than before the rebuild. I double and triple checked the balancer gear alignment during assembly, but in the end I suspect the crank is now out of wack. I was also very careful during assembly to not torque the crank halves. I had a local machine shop help pull the left main bearing off my crank and I bet they knocked it out of true in the process probably didn't use a proper puller. I say that because they also hacked up my left engine case when they drilled and welded back up (!) a small hole to push a bearing out I needed to replace. There must have been a better way. Point is, they were in a hurry and perhaps they weren't careful with my crank either and/or dropped it. That said, as much of PITA it's going to be I'll probably be tearing it back down to send the crank off for a truing job. GK (edited)