If you order a Mojave with the Steel Bumper Group, or if you get a Mojave X (which includes steel bumpers), it comes with stronger springs to support the extra weight. If you got it with a plastic bumper and want to go with the steel bumper, you should upgrade to the OEM springs that come with...
Looks like Idaho doesn't have a state-mandated cooling off period for car purchases. Does your dealer policies allow you to return the vehicle within a set amount of time?
I have 2 of the 67 Designs spring-loaded holders and they developed the rattle over time as well. But only when a phone is in the holder.
Using a MagSafe holder now to wirelessly charge, so no longer an issue.