OK, just an FYI.
did a little three hundred mile ride for ice cream in Winthrope WA.
Coming back started hearing this loud rattle at a certain rpm.
Got home and took off the skid plate.
I could see were a few rocks had made some contact.
At these spots it was close enough that at a certain rpm the vibration was just right to rattle on the frame.
Straigthened out and reinstalled problem solved.
Funny thing. It took a while after the Rock hits before the noise started.