What sort of noise, specifically, are you hearing? Is the hard drive "grinding" (that is, the sound it usually makes when accessing data), but louder than usual, or just more than usual? Or is it a different sound--thunking, whining, or something else.
If it's grinding more than usual, but not louder, then the problem could well be software related, or indicate that the drive is VERY full (which can cause application problems if it's the startup drive, though you can usually still restart ok). If this is the case, see how much free space you have--a memory leak or something writing a berserk log to disk or something similar may be the problem.
If it's grinding, but definitely louder than normal, I'd back up your important stuff RIGHT NOW (which, really, you should do anyway, even if there's nothing at all wrong).
If the sound is something else, like whining or loud thunking, you could well be in trouble, although a reformat rarely if ever fixes that sort of problem for more than a few hours, so it seems unlikely. In this case also backup right away, and maybe you should look for a new drive.
If you have AppleCare, run Techtool on it and see what it tells you. You could also try opening up Disk Utility and seeing if it says "S.M.A.R.T. Status: Verified" down at the bottom when you select the drive. If says something other than "Verified" (or nothing, which it says when it just can't read the drive), you're definitely in trouble. Or try the Apple Hardware Test CD that came with the computer--it has some chance of telling you something useful.
In all cases, only after a backup. ALWAYS backup.
