Ahh ok well original post sounded like carpet and plastic floor.Plastic floor doesnt really happen until u hit the floor under and around the spare wheel. No plastic as such until then.
If you take out both the carpet and its foam backing. and then the compressed clothes moulding to leave bare plastic,id bet its noisy
What difference a mat under the spare wheel and around the side edges of the plastic composite fuel tank will make, I dunno.
My 35 profile tyres threw up a few high frequency/low frequency noises.
The last loud(comparatively) one I discovered a couple of days ago.ie you could hear it above engine noise with the sound system off.
It was actually the rear seat belt clip tapping against the plastic stop knob on the seatbelt itself.
If there really is a lot of noise coming thru the 2 sound deadening layers at the rear :
It might be worth using real carpet felt underlay on the plastic.
Perhaps even pads of it between the carpet and its foam backing and the compressed moulding .
This stuff holds water so if the spare wheel well and fuel tank sides were metal,it might not be a good idea.
As it is ,carpet felt(the real thing) not spongy rubber might help without promoting any rust.
Is it possible the custom built exhaust is providing much more noise /resonance than the standard?