Another good tip is to make sure all your fans are the exact same brand and model #. You can even open up your PSU and change the fan so they all match. The purpose of this is to keep all the noise in the same frequency band, reducing the perceived loudness quite a bit.

If you pull out a 120mm fan, hook it up, and just listen to it on its own, you can experiment to see what's making it loud. Holding it up in the air, almost any 120mm fan is dead silent, because there is little friction on the blades, the air moves very smoothly from one side to the other, with very little effort by the fan.

Put some mesh behind the fan, and instantly it gets a bit louder. You get the noise of the air moving through the mesh, and the extra noise from the motor because it has to work harder.

Put the mesh in FRONT of the fan, and now you're really loud. For whatever reason, stuff on the intake side of the fan affects its volume output 10 times more than stuff on the exhaust side. Try it for yourself, you'll be amazed.

So naturally, after hearing that, you will be chopping out all the mesh from both sides (especially intake) of all your fans. If you need some kind of mesh, you can make it much quieter (and more effective) by spacing the fan away from the mesh and using a shroud. The distance of the fan to the mesh should be at least one-half of the diameter of the fan for best results.