I've had a MBP since April, and today, from the left side, I started hearing a loud roaring, almost a crunching sound, that did not have anything to do with disk activity, and is definitely beyond the "mooiing" and fan noise issues.
To be safe, I would take it in. Make sure your data is backed up. It could be the fan if the sound comes from the upper left corner. Further down is the HD.
Also, run the Hardware test. System disk 1, reboot holding down the d key. Choose the extensive test and see if you get any results.
I think i have the same problem. does it sound like someone scribbling a pen when you try to increase the volume. I just got my macbookpro today and im going to have to take it back!
For what its worth, I took my MBP to an Apple Authorized repair (do not go to an Apple Store in Phoenix unless you can wait hours in line at the genius bar), and it ended up being sent back for a fan and wiring replacement, and while there, got a new logic board.