I'm curious have you checked out the fan speed setting? I don't know if it's the same but my Antec 900 II case fans all have speed adjustment... get's fairly loud at max speeds but at low speeds it's quiet. What kind of noise is coming from it? Sound like a turbine or is it a rattling or something? Could be a broken fan/bad bearings. Like redechelon said, you can go pick up pretty much any 140mm fan and replace it.
Iam not much of a fan guy. Only the rear exhaust fan has a speed adjustment. Its like the turbine sound. I was thinking of replacing the 140 with like a 100 or 120. My PC exhaust is cold air. and the temps rarely hit 50 under load.