Another cause of noise is excessive dust/dirt on the fan blades causing an imbalance. For the little time it takes to clean with a Q-tip it may not warrant replacing. Also, I have encountered brand new fans that are noisey. Many people don't realize that oil serves more than one purpose (up to seven, as a matter of fact). Apart from the obvious lubrication aspect, oil serves as a heat sink and an impact absorber. Some new fans may have excessive clearance between the sintered bushing and the shaft. A drop of oil, in excess of the factory amount, will fill the excess clearance, minimizing vibration and transfering heat.
Tom Carson