Last weekend, I was driving down the expressway when I noticed a flapping noise underneath the car, as if I had run over a plastic bag and it was flapping in the wind. As I slowed, the noise lessened and then disappeared altogether. When I got home and stopped the car, I took a look underneath it and didn't see anything that looked like it could be causing the noise.
The sound didn't reappear during the week, but I only drove locally at lower speeds. But today, I drove on the expressway and the sound reappeared at highway speeds. When I got home and stopped the car, I crawled underneath it and saw a large sticker on the driveshaft that had come loose on one end, but was still stuck to the driveshaft on the other:
I pulled it off of the driveshaft and you can see where it was still stuck to the driveshaft on the end with the barcode and the loose end was flapping against the heat shield that covers the driveshaft:
This was a strange NVH issue, for sure.