First, check for something interfering with free rotation, such as:
  -lint wound around the fan shaft
  -dirt on the fan housing that rubs the tip of a fan blade
  -worn bearings, that allow the shaft too much sideways motion and
     let the motor armature rub the inside of the stator
     (Noisy, especially if you wiggle the fan in a plane containing its
     axis while it is running.)
Look for worn brushes (Under two large "screws" - one on each side at one end
of the motor.)  A shop specializing in small electric motor repairs, such as
electric shavers, might be able to find replacements.

These last two are only likely on older/cheaper fans, as most fans use
sealed ball bearings and brushless (electronic) commutation.

Even the best sealed ball bearings will eventually wear out.  Spraying
with oil will just make the outside sticky and collect dirt (which may
unbalance the fan).  Only if you can see the ends of the motor shaft
might a *tiny* drop of light oil help.  Use a toothpick or needle
applicator to put a drop of SAE 10 ("sewing machine oil") on both
ends of the shaft where it goes into the body of the motor
- unless you find oiling holes for an old journal bearing.

If the bearings are going, or there is a problem with the commutation,
consider the relative costs of the system and the price of a new fan.
A reliable fan is good insurance that you won't overheat and shorten
the life of your computer.

Boyd Ramsay

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