Based on your description, it's some sort of vibration in the engine/transmission. If it's doing it sitting still, you've eliminated any rotating part of the motorcycle/trike (which mean it won't be driveshaft, wheels, tires, axles, wheel bearings, etc). Also, pulling the clutch in also somewhat confirms this because the external rotating assemblies are still moving at the same speed, you've only dropped off the rpm's.
Items like this can be frustrating, and can come to a point that it's easiest to "ignore" it (One of my previous bikes was well known for a "buzzing" in the footpegs & bars at a certain RPM & was far worse if you changed to an aftermarket footpeg. I just dealt with it instead of driving myself batty).
You mention the bike is a 08 GL, but no mention of how many miles are on the motorcycle. The great majority of the time, things like this come down to maintenance in the end (heck, I've seen an air filter cause vibrations in motorcycles before). If you have the mechanical aptitude, you may be able to go down "the list" yourself & check off items one at a time. If not however, it can easily turn into a very expensive venture.