it's hard to say without hearing it...as HelloDeli said, all fans make a certain amount and type of noise and it varies from fan to fan. exactly which fan is it (i.e. case, cpu, chipset)? i know some chipset fans have a reputation for making noise and/or failing.
edit: the link that flood5 posted is very good as well. i have done this and i suggest you give it a shot as well...