Ailing Cube, insights please?

[Abigail Kelly]

My cube sounds as if it has a fan(?) that is
about to die. It is making a loud but uneven
whirring noise. Like when you take your foot on
and off of the gas pedal of a car. It doesn't
seem to be affecting performance but the part is
definitely about to quit working.

I would love to be able to replace it myself
since it is no longer under warranty. I also plan
on upgrading to OS X and reinstalling everything
just as general maintenance. I could use any
insight on the problem, the fix-it and the labor.

Thank you for your time and help!