Where's this noise coming from?

We did just move and dropped 2000 ft, so the carb does need a tweak. Would the altitude change require a timing adjustment too? I had it set pretty good a couple months ago. My tools are in storage, though. :tard:

Revving in neutral or park doesn't produce the noise. I only notice it while in motion. That's why I suspect it's happening behind the motor. I'll have to see if the noise occurs in gear with brakes engaged. If it does then I can capture the sound pretty easily. Any suggests for in-motion sound capture?