As suggested, I'd probably start with replacing the tappets. Mine were bleeding down and rattling after a few hours of sitting, with only 400 + miles on the bike. Generally thats when the lifters bleed down, is overnight or long periods of sitting, causing the rattle. Valve trains usually get a bit noisier after everything is hot, the oil is thinner, and the expansion/contraction of the metals.
Are you sure it's coming from the valvetrain? Sound travels through these motors and can be difficult to locate, especially when moving.
With that being said, make sure all external stuff is tight, and that the exhaust isn't hitting the frame or elsewhere. Maybe the next time the noise occurs, stop and get off the bike to see if you can pinpoint exactly where it's coming from.
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2015 Denim Black RG, FM ss ceramic coated 2-1-2 pipe, White Bros Original E Series Mufflers, FM ac, PV Tuner