VW new Beetle 2004

I just purchased a 2004 new Beetle. It has good back to the Lot 3 times, they have no idea, it went to VW for 2 days no answers…Help When you step on the brake, there is a noise that comes from the trunk or back of the car. The sound is like a window shade in the house been let go when it rolling up. This is not everytime you step on the brake, every 3rd or 4th time. We have taken the spare tire and jack out nothing, So If someone has an idea please let me know. That or we have a former owner staying with the car LOL

Does it happen at any speed and even standing still?
In other words, if it happens when the car rolls slowly enough, can you try to pinpoint it from outside the car while someone drives it.
Could the muffler hanger be loose and the muffler just flops around when you step on the brake?