Motor speed does not affect it. Bike speed does. Naturally it is more noticable when the clutch is pulled and bike coasting wether in gear or nuetral because the motor is quiet from being at idle and the bike moving. Thats why i ask if this is normal noise on all rockets. Sounds like junk to me.
Ah ok, so that's quite helpful in isolating it to the trans. Is the sound the same pitch/tone in all gears with only speed causing the variance? Just trying to help isolate it for you.
If it's the same noise when in any gear with clutch pulled coasting and the only thing that changes it being road speed, that limits it to likely being the secondary reduction gear, torsional damper, damper bearing, or shaft/bevel box.
Some easy things you can do to eliminate a few of these things and narrow the focus. Since you've just done 4k in wet weather these are a good idea anyways.
Change the bevel box oil, it's only a small amount, be careful and use the torque specs so the drain bolt doesn't snap off when tightening, 15 inch pounds.
Lube the shaft splines at the bevel box input, use the manual when retorqueing the 4 done nuts that hold it in place.
Inspect the shaft U joint. It's under the rubber boot that sits at the back of the engine, left side.
If none of that effects the noise at all, and the Ujoint looks ok, take it to a Triumph dealer and let their Rocket 3 tech ride it. See what he says about the noise. The trans does whine a bit since of the gears are straight cut.
If he confirms an abnormal noise you'll be searching inside the cases a bit, at the torsional damper and bearing. These repairs aren't expensive parts really, but involve a good bit of labor.