Gearbox Cause of Vibration?

All I can say is that I had very similar symptoms, vibration when throttling back. The engine ran smooth under load or at high power settings but there was a pronounced vibration under light load.

The problem turned out to be excess wear on the splines of the prop shaft in the gearbox. I had to have the gearbox rebuilt with a new drive dog and prop shaft. The Rotax specialist said that he had recently seen a number of gearboxes with the same problem. During my ownership the engine was never idled at a low rpm so I don't know what caused the problem, it could be the low mass composite prop.

My current engine has a slipper clutch in the gearbox and so far it has not suffered from the same problem even though it has now done more hours than the original engine.

I don't think that you can diagnose this problem without removing the gearbox and disassembling it. You need specialist tools to do the latter or someone who can do it for you. It shouldn't be too expensive to have it inspected, you could just bring forward the scheduled gearbox inspection.

Kevin - Did your gearbox make metal chips on the magnetic plug?  Sure looks like it must have....

It was not obvious, there was only a very small amount of very fine particles on the mag plug at each service, certainly not enough to cause concern.

I think for spline to wear would be a metallurgy problem, class of fit, surface finish, or improper heat treatment.  When you sent gearbox in for inspection, did the shop return your old parts for your inspection?

Have you considered your engine mounting/isolator rubbers may be fatiguing?😈