I posted something like this about my AWD CC several months ago. After MUCH ANNOYANCE we've sort of figured out that it isn't the suspension as first believed, and isn't the fuel tank, but rather something rubbing in the back seat that I haven't had a chance to properly investigate (I'll have to sit back there while missus drives over a bumpy road).
I suggest you get under the car (safely) and push / pull things to see if you can find the source, particularly after a drive. At one point I believed it was my fuel tank coz that makes some noise if pushed in the belly of it. Ideally, get it on a drive-on hoist, with someone bouncing on the door sill with their bodyweight nearest that corner while another is under listening. It can be a real hassle to find.
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