Fan is a 6k RPM PWM 92x38mm, but keep in mind all device programing is based on this fan...if the hardware monitor detects RPMs out of range it will auto-shutdown the miner. Fan monitoring is pretty lax, and you can probably get away with a 3-4k RPM fan but in that case it probably wont be able to cool past balanced mode (there is still a over temperature shutdown for that case if the fan can't keep up so you should still be fine).
I overbuild all my products for the worse case scenario, and the fan/heatsink has to be able to keep up in a wide range of temperatures/environments so while the fan is pretty quiet in eco I could have obviously gone quieter but sacrificed top end cooling ability.