Like anotherbrian said, the vest fits tight. A race fit. There is no room on the sides. I like the fit except there is no vent. When I am riding with my back almost flat, it flaps just enough to make that dreading flapping noise.

According to my observation, this flapping issue is caused by the lack of vents and the fact that the arm holes are relatively stretchy. It is enough for air to get inside through the arm holes. Behind my head is a vacuum. Due to pressure difference, the air naturally flows toward behind my head, along the shoulder. However, since there is no vent to allow the air to escape, it lifts the vest along my shoulder and escapes through the neck hole.

My plan is to make holes along the shoulder line (say 10 holes total) using a heated precision pick and pierce the vest to relieve a built-up pressure in the vest. Hopefully, well-placed 10 holes are enough to balance the flow. If the rate of air flow coming in > going out, I might need more than 10 holes or bigger holes. At the end of the day, it is a $35 (price of the vest) worth of experiment.