Re: 2014 Trailhawk rattle
That transmission valve is also unattached in mine but that was one of the first things I secured. I connected it to another point away from the firewall. The heater lines don't cause any vibration in my Cherokee, although I've read that it does in other Cherokees.
In mine the vibration is positively coming from the A/C lines in the firewall. We've verified that by removing the zip tie and foam and driving it around. There's a rectangular hole in the firewall for the A/C lines and a rectangular fitting sits in there with about 1/8" wiggle room. There's supposed to be foam around it but on my Cherokee you can't even see it.