Vibrating or rattling noise type noise.

[09 Softail C]

I have a 2016 Street Glide with the 110. It has a vibrating or rattling noise I believe is coming from the primary chaincase. I can feel the vibration when it happens with my hand out the outer case. It will happen when idleing if the RPM's drop a little, its not consant so the rattling will be brief. Idle surges between 990-1030 +/-. It can also happen when I take off if I'm not giving it quite enough throttel and the RPM drops a little. Any ideas what this may be? Bike just turned 13,000 miles. TIA