I'm trying to put a second fan inside my Korad to cool the transformer as it is getting quite hot. The fan i'm using is a Delta AFB0712VHD, voltage range 5v-13.2v The Korad's fan controller outputs 6v idle 12.03v full power. But i noticed that the Delta fan rattles when it idles at 6v, any idea why is that? It works perfectly when the fan speeds go up and when it drops down to idle it rattles. It shouldn't be vibration as the fan does not physically rattle, it just makes a rattling sound.

The stock fan that came with the Korad doesn't have this problem.
When i power the Delta fan with the Korad's output it spins as low as 4.3v before it stalls.