The keyboard area itself, or the palm rest below the right side?
If it's the keyboard area, it's probably a fan, and if it's the palm rest it's the hard drive. Does it do it constantly or intermittently? You could probably take it to a Genius for a fix if it does it constantly.
I say that because if it only does it intermittently, you just KNOW that it won't purr for the Genius when you take it in. They're shy like that.
I just downloaded SMC and ramped the fans up but no change in the sound. It only does it intermittently. (Knowing my luck it wont purr when I go a Genius)
I think I will wait to see if it gets worse before I drive miles to the nearest Apple store. If it is the hard drive and its going at least I have Time Machine running.
Until then I will just play music to cover it up Thanks