You were right about the joint being the problem. It's the one located above the X on the top rail (long side). If I hold the joint with my hand and depress the throttle, the rattling stops.

I tried adding spacers. No luck. I tried installing a clamp with a grommet over the bad joint. No luck.

Rad Power Bikes San Diego was incredible. They shipped me a new one. No rattling.

I just installed the large basket on the back of my Roadrunner 2 and now there is a fairly strong rattling sound every time the motor engages.

Anyone else have this problem? Any solutions?