Why is this airduct twitching and making a noise?
The outside end of the duct has a flapper to prevent air (and critters) from coming into the house when the dryer is off. However, even with the dryer off, there can still be a small amount of airflow through the duct, e.g. from inside/outside air pressure differences. It seems that in your case there isn't enough airflow to keep the flapper open, so it swings closed, making a "click". Then, as the (small amount of) pressure builds up, the flapper pops open to let a burst of air through. The changing air pressure causes the flexible duct to throb. Repeat twice a second, and bingo: alien living in your vent duct.