So I did this just before my trip away, but didn't get a chance to post it.
I removed the back seat to aid in running cables to the boot (for my fridge setup). I figured while it was out, that I would add some sound deadener to the rear wheel arches.
I cut some panels to size, then held them in place to mark where mounting studs were:
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Then I used a gasket hole cutter, just by hand, and twisted it through the aluminium and butyl layers:
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Nice neat holes:
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And fitted into place, with the OEM compressed fibre layer back on top of it:
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And then I added a layer of open cell EPDM foam to the back of each side panel, against the hard plastic layer:
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As well as that, I also shoved some "pillows" of puffy fibre/fabric in behind each trim panel in the boot, and pushed them up over the wheel arch area as far as I could.
It made a slight difference on the concrete sections of the freeway. Enough for my wife to notice there was less noise from the back.
It made no difference on the "chip seal" crappy road surfaces that are just spray tar with gravel on top, which is very noisy.
On the new smooth bitumen, made with recycled tyres, it's almost silent - the A/C fan is louder than the tyre noise.
I will be doing a lot more over time, but just did that while I had the access with the seat out.