So I ended up experimenting with ATF to reduce the noise... here are the results so far...
The motor noise is about normal for a year-old BPM, since Ben kindly let me ride his bike for comparison. Mine isn't hideously loud or anything, but I am convinced there is a bit more whine than when it was new, and I do like to be stealth! So, after reading a couple of posts about ATF making them run quieter, I removed the grease and added 50mL of ATF. Thanks to Marc S for his advice. About 100km later, I removed the awful gunk mix of ATF and disolved grease, and added 50 mL of fresh ATF.
Motor efficiency seemed to be reduced by ~25% initially (or maybe I just got lazier for a while??) but now it seems back to normal. And the noise? Yes, it is reduced, it seems to be back to the volume of the motor when new.
But I forgot just how evil ATF is :lol: It gets everywhere! It is weeping out of the motor cover and the back half of the bike has a thin sheen of it. Yuk. I might experiment with better ways of sealing the motor cover, but I suspect in the end I'll go back to grease. With a new clutch and gears (that d8veh linked above- thanks!) to make it quiet. Since ATF disolves grease, the issue will be getting the motor thoroughly cleaned of ATF before putting it back in with grease.