I just bought a 2006 XC90 Ocean Race Edition. When test driving it, it drove fine except the brakes weren't very good. When braking The vehicle would shake and a bit loud. Anyways, I drove on the highway today for about 350 miles and it was fine. Then I hit traffic and slowed down to a few miles an hour and it started. I can't feel the knock but it sounds like it's coming from the rear. It doesn't matter if I'm going straight, turning, in drive or in neutral it always goes with the speed of the tires. I looked under the vehicle quickly and couldn't see anything loose. Any ideas?

 

Read the attached thread. I had the same issue and it was the center cv joint. Although "not serviceable" I did service it myself and so far all well.
Knock on wood 25k later.

https://volvoforums.com/forum/volvo-...eration-66087/

Good luck.