When I’m accelerating from around the 1000 to 1500 rpms hard there is a brief chunk clunk sound. It seems to be in the top of the motor. I do not get the sound if accelerating slowly nor does it seem to happen at any other time but from low rpms. Anyone have any ideas

 

Without being there to hear it I'd have to guess that it's the clutch making a little noise. Does it give you a little surge forward as it makes this noise?

 

The motor is hung as a stress member, maybe you should check your motor mounts and your drive chain slack..
If these are not correct the low rpms ( most likely low speed) is heard because the most force is being used to get the bike moving

 

Yes its the clutch and its totally normal!

Try pressing the clutch handle and the noice will probably disappear.