My hexhead adventure does it. I tried covering the clutch pushrod with heat shrink insulation. It made all of no difference whatsoever.
I've had the gearbox off and found nothing wrong so just put up with it.
My hex head has a new clutch and still rattles in neutral until I pull the clutch lever in. My money is on end float within the gearbox, it gets worse the hotter the engine/gearbox gets but still shifts gears smoothly.
I read somewhere that it's gearbox end float. Mine rattles the same with clutch push rod taken out so its not that doing the deed.
Options -
(1). New gear box and wait for that to start rattling
(2). Fit new bearings and sort the end float in your existing gearbox
(3). Wear ear plugs.
2 has some merit - search "dodged a bullet in gearbox".