before you replace anything .... look inside ... if the fan looks like a fluff ball ... give it a clean.
as bu asks
is it "hunting" or whining ?...
hunting tends to be a harsh mechanical clicking
if it's bearings ... the easiest way is to touch the disc with a screwdriver and get your ear on the handle
fans sound like vacuum cleaners
let
http://www.hdtune.com/
have a look ... look at the health tab
if SMART shows errors ... it's backup time.
get the good stuff first ... if it fails ... switch off ... wait a while ... try again