[Xult]

What I would recommend is to search Newegg.com for a good deal on a more quiet fan to suit you. Otherwise if your fan is loud check it for dust because usually the fan will try to work harder heating up faster with dust on it. If you do have dust buy Canned Air to blow this dust off the fan and it should work better. But once again if your fan is just loud go to option 1 and just purchase a new one if it really bothers you.

[DataSaurusHex]

WD-40 should help. bit of sparkle to it and is good

[apetrov]

Machine oil and perhaps a dust cleaner can do miracles. That is if the fan is old and used for a couple of months.