This guide will tell you where to farm for Stalking Fan in Warframe.
Stalking fans can drop from certain corpus enemies and as a completion reward for completing Rathuum.
If you’re going to farm the mod through Rathuum, you have to run the Nakki mission. The mod has a fairly low drop rate, so it will be a bit of a grind to get the mod, no matter which farming method you choose.
You can farm the mod by killing vapos or normal nullifier crew men and vapos sniper crew men or just normal sniper crew men. You can find vapos crewmen on Jupiter, and the normal crewmen can spawn in any other Corpus mission. But it is recommended to farm them on Baal on Europa, and for vapos, you can go to Elara on Jupiter.
The two enemy types you are looking for usually start to spawn after about the 6 min mark 10 if you are unlucky. So you will have to stay in the mission for some time before the enemies start to spawn. The enemies you are looking for for the exterminate mission will spawn randomly throughout the mission in either high or low numbers.