I hear a strange UFO like, bird like chirping noise coming from the rear of the car. The chirps increase and decrease in frequency but with no regularity and do not match engine rpm or vehicle speed. It will do it at idle or highway speed but gradually disappears. It comes and it goes when it wants to.
Maybe someone else will have special insight into that vehicle, but the only thing back there I know of is the fuel pump…but I’ve never heard one chirp. 
Suspension a possibility? If engine RPM and vehicle speed don’t play a part, does the condition of the road? Does it do it when you’re sitting still? If so, it may take some looking around to find it.
It’s a possibility. Watch this space for more great ideas.
Chase
I would look for something metallic thats loose
An L2 cetified VW mechanic says its probably a bad leak detection pump, a device for monitoring whether things are sealed up in the gas tank, cap and lines. Anyone agree?