From what I can tell it's all tire noise, the back cargo area isn't insulated against noise very well. When I bought mine folks on-line complained about the original equipment tires, when I bought new ones I could tell the noise was less. It's not terrible but at 65+ mph on the freeway you have to turn the radio volume up a bit to hear. Less than that it isn't an issue. Keeping the back seat up mitigates that but I usually keep mine down since I use the full cargo area most of the time.

Gunnery, gunnery, gunnery.
Hit the target, all else is twaddle!