Why is it that the single flywheels like the NV-5600 chatters under 1000 rpm? What is actually happening?

 

The Cummins produces low-speed harmonics which are quite noticeable at or below idle rpm; the dual-mass flywheels were specified to eliminate the occurence of the undesirable harmonics at any operating rpm, however they are still evident in ~ 400rpm.

Obviously, some aftermarket clutches are known to cause chatter upon engagement due to their friction surface materials (ceramic, for example)

 

So it's almost the plate on the engine bouncing a little?