Work out where the noise is coming from first.
Suggest a few blankets put over the rear wheel arches would be a good first move for research, then if that improves things try and pin down other areas and cover those with blankets too, an assistant in the back doing this as you travel along a quiet fat road would be ideal.
When you've isolated the noise, then if you take the plastic trims off chances are you will find no soundproofing at all between the trim and the bodyshell, you can buy self adhesive insulating pads on the usual auction site (8 or 10mm thick ideal), trim to fit and apply.
Also check under the rear seat and in the boot area, a couple of hours and £50 on sound absorbing materials will make a big difference, you can go further by removing the door cards and applying sound pads to the panels, front bulkhead another possible, but by the time you've insulated wheel arches and boot and under seats areas you'll have lessened the racket considerably.
It possibly also the tyres are particularly noisy, if so that's an easy change when the vehicle needs new boots.
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