Replaced fans with Noctua. Why do the fans keep spinning up and down?

I followed the guide to replace the fans with better quality fans and went with a pair of 92mm Noctua NF-A9 fans. I opted to desolder the original fans, snip the connector off the Noctuas and solder the positive and ground onto the original fan cables. The procedure went smoothly but now that everything is reassembled the unit keeps oscillating the fan speed from low to high. It sounds like it's trying to kick the fans to a higher speed and then immediately goes back to the lower speed. What's weirder is that eventually it will lock into the higher speed for the fans but seems to react when I put my hand near the intake vent and go back to the lower speed. I've experimented with it enough times to be absolutely sure that it is reacting to my hand being put near the intake vent. It acts like there's a sensor there detecting my hand, but it's more likely that it's reacting to (very tiny) pressure or airflow differences. It's not like I'm obstructing the intake, just putting my hand maybe .5-1 inch away to feel for airflow. Maybe it's a power/voltage issue?

The stock fans show 12v 0.2A and the Noctua's spec sheet lists 12v 0.09 so they should be requiring less power, right? Now sure what to make of any of this.