may sound silly but is there not anything loose in the boot? i had this happen had me scratching my head for a while
Mine has this. Turned out that my rear wheels were rubbing on the shocks, have a slight bend in the rear beam.
Check the shock rubber covers for any rubbing.
Check your rear damper bolts, top and bottom. I had a knocking noise last week turned out to be loose bolt on the bottom damper.
Also check around the inside rims of your alloys to see if the small weight isnt catching on the inside of the arch.
I've also noticed my exhaust seems abit loose. Could it possibly be this?