Clunking/rattle from rear when going over bumps in cold weather

I have a 2010 CX9 GS AWD, about 5500km on it right now and about 5 months old. I live in Canada, so it's gets pretty cold here. When it does get cold, below about -20 degrees celsius, my vehicle makes an annoying clunking in the rear when going over bumps. I took the car to the dealership twice and they said it was coming from the rear hatch and they put some foam in there to dampen the sound, but it hasn't helped at all.

To me, it sounds like the noise it coming from somewhere above the wheels in the rear. I asked the service adviser if it could be the shocks and he told me it's not that. I however think it is the shocks, I think the fluid in the shocks might be freezing and causing the clunking.

Has anyone else had this type of problem?

Probably the rear struts need some grease.
The top side is very close to the rear hatch.
I can see why someone would think the noises come from the hatch.

That's not a normal sound for normal temps....but if you think about it, -20c is really, really cold. Rubber becomes almost rigid at those temps. If you look under your rear wheelwell, you will see a rubber damper where the strut meets the top of the wheelwell. That rubber damper is probably rigid in those temps.

Do you have anything in the rear storage compartment? I have a similar noise going over bumps but I keep a towing receiver bar/ball back there and it just bounces sometimes making a klunk.