Check the rear seat backrest latch mechanism - a common source of rattling (tapping?) from the rear of the car’s cabin near the rear door / seatbelt area in many VW (and VAG) models; I’ve had it in previous VW’s and so have some members on the forum.
Often, there’s free play between the latch in the seat backrest and the metal loop that the seat backrest latches onto when it’s in the upright position. A quick, cheap and easy fix is to wrap some electrical insulation tape around the metal loop to remove any free play. If the tape does wear through from repeated lowering and raising of the seat backrest then just replace the tape with a new piece.
