The backseat area is always going to be noisier than the front...just like most vehicles. In the Equinox/Terrain specifically, it's because you're basically sitting on top of the rear wheels...and you don't have the laminated acoustic glass like the two front windows/windshield. Also, the cargo area can act as sort of an open acoustic chamber that can amplify road noise on certain types of pavement. I think GM skimped on the wheel well insulation for these vehicles either for weight or cost concerns...hopefully somebody will figure out a fairly easy DIY method using some kind of Dynamat material..
You have a 2006. The newer versions 2010 have as mentioned before an Acustic glass winsheild and laminated front windows ( The side windows compare to your 2006 winshield). They also have other sound dampaing features such as more insulation. The 4 cyl has a noise canncellation system that uses sound to conteract road noise.
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