Generally the best way to get a quieter fan at the same flow rate is to use a larger diameter fan because a lot of the noise from fans is aero and a larger diameter means you can have the same volumetric flow rate at a lower velocity, but that isn't really practical advice.
Things to try to quieten a mac fan include...
1) (as mentioned) oil the bearing
2) clean the blades (dirt/dust on the blades causes an imbalance that makes them louder)
3) clean the case inlet and outlets - reduced cross section increases the peak air velocity
4) replace with an identical new fan
5) replace with a similar or better new fan
5 is difficult - fans sold as "quiet" are generally designed for low back pressure use cases, and the quoted airflows represent unloaded performance, and the quoted backpressure represents the stall scenario. You can ballpark the performance by drawing a line between these points on a pressure / volumetric flow plot, but this doesn't account for the actual characteristic.
Most fans have sort of a... flat bit... in the middle of the curve. You need to avoid operating in that area because it causes significant speed variations with small changes in pressure (i.e. due to draughts or whatever in the room). This reduces efficiency, increases wear, increases noise... and is irritating. See "Normal Working Range". You want to be on the steep gradient.
Most Noctua fans are not designed for this type of use case.
I strongly recomend maintenance or like for like replacement. But I shouldn't really be commenting because someone always has a go at me whenever I mention fans.
