2017~2021 Crazy Noise from under the car and the cause!

2017 Cx 5 GT Premium
For the last two weeks, I have been going crazy trying to find the source of a rattle in my 2017 CX5.

I sounded like the jack was loose under the tire in the wheel well. (The jack sits in a styrofoam tray, not cinched in a bracket in the hatch, as many cars have.) After shaking and testing pretty much everything in the hatch, under the rear seat, and under the car, (mostly focused on heat shields), a local mechanic banged on the lower A-frame member and heard something. Turns out there were a 1.5" rocks in both the left and right sides. Probably thrown up after getting stuck in the tires. Although the A-frame member is open on top you couldn't reach the offending stones. The bottom only has two small holes through which to maneuver the object out.

It was quite an unusual solution but there you have it. No more noise.
2020 CX-5 GT
Interesting. Have you done many miles on gravel roads? I've probably put about 300 miles on gravel in my CX-5 this summer but fortunately no rattles like this one.