I think it's a 40mm chipset cooling fan that's available from most places that sell computer parts (Micro Center, New Egg, Tiger Direct, Fry's) for $7 or so.
Normally the fans are very quiet but they do eventually wear out. Especially the cheap ones with bushings that are used on these mini mills. The better fans have ball bearings.
Chris Z.
From: "bbass408" <bbass408@yahoo.com>
To: GrizHFMinimill@yahoogroups.com
Sent: Wednesday, January 25, 2012 5:42:49 PM
Subject: [GrizHFMinimill] Anyone ever replace the noisy cooling fan in the controller box
I know I'm being a whiner but that cooling fan has a nasty startup noise. It has always done that and always calms down after a few minutes. But, it's so easy and inexpensive to replace (I'm guessing) that I thought I might try a new one. Does anyone have any recommendations? Any info on any interesting specs would be great too -- size, bolt pattern, CFM if that really matters, and voltage (which I think is 12 based on other posts).