[missile]

Sounds like a loose electrical connector or wire. Reminds me of the (true story) of the guy who bought a brand new Cadillac. He was driven mad by a rattle, had it back to garage many times before they removed the door card and found a bolt with a label " found it at last you lucky rich b...... "

 

[missile]

I would remove the C(?) pillar then the rear parcel shelf and test drive

 

[missile]

I would try removing parcel shelf first. I had a squeak from mine.

You might want to open the side compartment in the boot and remove the stuff in there then check for loose wiring etc.

It is quite easy to remove and refit pillar, but safer to get Audi to do it.