[DrivingPassion]

Well, I got my wife to drive it around with the rear seats unhooked and completely flat.
I laid on the flat rear seats and I could pin point the sound coming out from under the right rear speaker. But, there is an entire metal casing enclosing the speakers with 4 large bolts.

I guess if I'm itching to find out why, I can remove the 4 bolts and see what happens?

And, yes, I agree, it is temperature dependent. If it's 90 degrees F and above, it is easier to reproduce the sound than when it is under 70 degrees.