I think it's dependent on the motor revision, as some do and some don't rattle, let's hope so, cause if all motors develop the rattle after some time that's a bummer, but time will tell right.This reviewer also says there is rattle with the avinox. Its not just frame specific. Some, not all Amflow, Forbidden, and Crestline owners have experienced some type of rattle.
One of my thoughts also was, what if the rattle discrepancy between motors has to do something with how much grease is packed between the internal gears, ratchet, etc...
If there is little or no grease the gear backlash is definitely easier to produce then if there is a generous amount of grease in there.