It's pushing through the heatsink. I've tried multiple methods for the rest of the mounts, and none of them really seem to make a difference, hence how I got down to three totally necessary case fans. The addition of one on the back, whether intake or exhaust, added nothing but noise, and didn't lower my temperatures at all. Neither did a second 140mm on the top or bottom. I haven't attempted to mess around with my heatsink fan since I got it, primarily because I've been within the acceptable temperature range with all of my components, but it's definitely something I could try in the future, though I don't see what difference it would make. I figure at any rate the air is coming from the back, and being removed by the fan immediately on the other side after passing through, which would be better than it fighting my sole exhaust for airflow.