We've come to the conclusion that fan manufacturers ship a mixed bag of units. I have been buying fans in pairs to try them out. Sometimes you get a fan of a particular manufacture and model that has noisy bearings, and another which doesn't. Also, the amount of lubrication may vary from one unit to another for the same model, manufacture and factory.
You can spend some time troubleshooting this noise problem, or hope to choose the shortest distance between two points. Some time ago, I was able to acquire a cheap stethoscope to use in tuning my Honda's engine, and it has come in handy with the computers as well.
A lot of fans may have motor-noise to some degree or another. If they can't be lubricated into silence, it may help to use silicone-rubber grommets to keep the noise from being transmitted throughout the case. If you don't want to pay for grommets of commercial manufacture, you can obtain a tube of silicone adhesive-sealant from an auto parts store, remove what you think is the offending fan, put a thin bead of the silicone goo around the edge of the fan frame where it will make contact with the case, and let it set for several hours. Then reinstall the fan.
I think -- from personal experience with a CoolerMaster Wavemaster -- that aluminum cases are a bit like tamborines and snare-drums: sound bounces around inside them with abandon, and the aluminum panels have a particularly tinny ring to them. So another (cumulative) solution to the noise problem might be the purchase of $12 worth of Akasa PaxMate noise deadener pads. You don't have to fill the case with it, but -- assuming you're not going to cut blow-holes in the side of the case (which by the way will increase the amount of noise transmitted into your work area) -- you might want to cover the side-panels with the stuff.
As corollary to this, if aluminum cases tend to amplify certain types of sound, you have two remedies worth pursuing: eliminate the sources of the noise, and eliminate the sound-reflective or sound-transmitting properties of the case itself.
And be careful about lubricating fans. I have it from reliable sources that adding a little grease appropriate to the type of bearing used in those fans may eliminate some or all of the motor-noise, but I also have it from reliable sources that if the lubricant is the wrong type, or a lubricant that "migrates" -- you could have a mess inside your case and on circuit-boards that will attract dust like a magnet.
