Most modern cars have a fixed lining in the wheel wells, it's designed to breathe and allow moisture to escape, it's sort of a soft nylon snap in liner that absorbs the gravel/sand noise that would hit the wheel wells and cause cabin noise.
Unfortunately, in the Z rear wheel wells, Nissan decided to forego spending 50 cents to stop the noise in our cabins. Adding anything other than that I feel won't alleviate the problem.
In my Z I added Dynamat-like soundproofing throughout the rear cabin, but on the passenger side of the vehicle the inside surface of the rear fender isn't accessible, (due to gas line passage) and it rings like a drum when rocks and gravel hit it.
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