The torsional resonant point is highly dependant on how smoothly the engine idles.  If you have no misfires, the RD-1C hits resonance at around 800 rpm.  If you have any misses at idle, this immediately introduces a much higher amplitude and lower frequency excitation to the system and it will show up at a higher rpm.
 
In summary, the "rattle point" is more dependant on idle quality than absolute rpm.   Do not be too concerned about this however.  It will cause no harm to the drive or engine.  The drive has been torture tested for over an hour running on one rotor over the full range of throttle settings.  If you've ever run a twin rotary on one rotor, you know how violent this is.  George Graham ran his Mazda transmission redrive on one rotor for about 2 seconds before every gear tooth on one gear was stripped off.  Tracy  (now canoeing to the post office one last time before departing for S'nF)


    Greetings,
         I asked this a couple times, but it was always buried at the end
    of long messages, which gives me a clue how few people make it to
    the end of those :-)
         IIRC, the RD1 drives have a low rpm point where they're known to
    rattle.  I guess this is torque reversals, or something like that
    from the engine.  Anyway, can someone confirm that this is true,
    and tell me what rpm this is supposed to happen at?  I've never
    been able to idle below 1500 or so, but now, I could probably idle
    down to 1000 easily.  The problem is that I seem to hit this
    rattle mode at 1100-1200 rpm, so I've got my idle set at 1400 for
    now.          Thanks,
    Rusty (wondering if I should have inspected the redrive following
    the oil out)
              PS- TIG and Plasma machines both got 1.5 inches of water before I
    picked them up, but I dried them out yesterday, and they both work
    now (thank goodness).  Fortunately nothing expensive was damaged
    in the flood, assuming we don't have to replace the carpet in the
    great room.