Which type cooling fans why prone to noise?

Cooling fan in accordance with the classification of bearing, mainly oil bearing and double ball bearing two kinds big, but it seems that oil bearing type cooling fans are more prone to the problem of noise, is this why?

oil bearing materials is poorer, rough process, precision is poor this is caused by long time use oil bearing type cooling fan noise, easy production beginning sounds very good, because between the bearing and shaft core has the effect of lubricating grease, slows down the friction between the core shaft and bearing directly. But as the growth of the time, lubricating oil will kill, the voice is more and more big.

and the precision of the double ball bearing is very high, is steel, from the beginning is what kind of sound, and the same voice a year or two and it is also a lot of use for a long time, and the product with high requirements for noise, using double ball bearing fan instead of oil bearing type cooling fans.