Hi All..... my XC90 performs faultlessly in every respect.
No knocks, bangs, squeaks, or any anything untoward from the engine, gearbox, suspension, brakes, diffs etc.

I have taken the car offroad a few times (Sand) and there is no issues with the AWD.

BUT..... There is a resonance that fills the cabin when i decelerate. It only happens @ 1200rpm to 1800rpm and between 75km/ph and 90km/ph but peaks @ 1500rpm and / or 80km/ph.

It fills the cabin but there is no vibration to be felt, just this loud resonating buzz (not a wine or a vibration) and it makes the kids feel sick when on our daily commute i have to sit for 5kilometers, going downhill at 80kmph @ 1500rpm.
If i take the car out of gear it still does it and if i start accelerating it stops.

I have checked all the heat shields that run under the car and they seem fine, there doesn't seem to be an issue with the rear engine mounts.

Anyone else found this problem? If so, what was the solution? Car is a 2010 D5 Executive with 7 seats and running Pirelli Scorpion Verde tyres.

 

Check the differential mounts.
Check transmission mounts (prob not this if it makes noise out of gear though)
How are your tires? Even wear? Getting low? Could be you rolling on wear marks even so slightly?