Its possible the ball bearings on the case fans you have are going out. It will make alot of racket when that starts happening.

Only other thing that would be likely is your hdd, but you would be noticing other things happening as well if your hdd was starting to go out and make enough noise that you could hear it. Its more likely that one of your fans is starting to die, which isnt all that uncommon, especially if they use cheap fans.

You are talking at best, $15 to replace the fan, and thats including shipping.