Remember this odd little creature?

It used to show up in the "Page Orientation" dialogue whenever you went to print anything on your Mac. Because of its rather indeterminate appearance - contained both bovine and canine characteristics - it was dubbed the "dogcow"...

And now, it seems to be making noises in some peoples' MacBooks - specifically, from the fan.

As you know, the fan kicks on and off in its unending job of keeping the CPU from overheating. Some users have reported a "moo-ing" sound emanating from the mechanism whenever it performs its task. One user took his MacBook to an Apple service center only to be told, "It's normal. A fan will do what it needs to do to keep the machine cool." 

DUH...

Apparently, the MacBook Pro also had this problem, which disappeared when the fan unit was replaced. Obviously, a MacBook that goes "moo" is NOT normal.

Your best bet to exorcise the dogcow...? Call AppleCare.