Diagnosis
The smokestack on the Highland is a slip-fit into a hole on the cook chamber. There's no locking mechanism, so wind or vibration can make it chatter in its socket.
How to Fix It
The simplest fix is to get a small metal hose clamp (worm gear clamp). Open it up, wrap it around the base of the smokestack where it meets the cook chamber body, and tighten it down just enough to pinch the stack slightly against the side of the hole. This takes up the slack.
If That Didn't Work
If the fit is really loose, you can add some high-temperature RTV silicone (the grey stuff) around the base of the stack before inserting it. Let it cure fully—this acts as both a gasket and an adhesive. The clamp method is easier to reverse, though.
Heads Up
Don't weld it or permanently attach it unless you're sure you never want to remove it for cleaning or transport. Also, make sure the damper on the stack itself is also tightened down—that's another common rattle spot.