All fans get dusty/dirty overtime so clean with a brush to loosen that up and free it for best operation. Since, its noisy as well though, you should consider a replacement before its too late. However, if you're handy you can give new life to it as follows:
If you have a noisy fan and no replacement, try the following. Remove fan from mounting
and save screws or unclip the fan. Clean any dust or dirt build-up at this time as best as
possible. Now with fan in hand, most fans have a sticker maybe with some logo, this is
the top(exposed) side, the bottom side sticker, probably blank. The bottom side is the side
you want to work on. Slice a small half-moon cut, slightly off-center on the sticker/decal
on the bottom. You *don't* want to remove sticker/decal entirely. Flip flap open to expose
center of fan and place 1-drop of oil there, don't over do it, blot any excess oil. Rotate the
fan to work the oil in and then place fan with bottom side up on a flat surface. Allow
gravity to migrate oil into bearing for 10-minutes.When done, tape over the cut flap and
remount fan. Use deft care and don't over-tighten screws, you're done.
While the system case is open, clean the rest of the system out and don't upset any connections, etc..
good luck -----Willy