Seems like a rattly tailgate/rear hatch plagues not only the V70s, but pretty much any hatchback. Anyways, just thought I'd share what I did to eradicate that annoying rattle.

The tailgate lift supports were shot, so I replaced those with Volvo OEM ones. Also installed one of those tailgate trim clip replacement kits from IPD (reinforces clips on the inside of the tailgate panel. And after all of this, still the rattling. Tried using pieces of thick leather at the PS and DS rubber contact points on the tailgate, all that did was make the squeaking higher pitched.

So, I ended up adjusting the bracket that the tailgate latches on to. It is held on by 2 torx bit bolts (probably about a torx-25). I loosened them up, and moved the bracket towards the front of the car, to make the tailgate close tighter. I moved it by less than 1/16th of an inch. Make sure to keep it centered from where it initially was (the marks of its initial placement should be clear). And that took care of all rattling coming from the back of the car!

Super stoked to have one less rattle on the car. Hopefully this helps!