You might be hearing the hard drive. You should be hearing a gentle whirring noise accompanied by periodic clicking. This is normal. You may hear it clicking when you're loading a program, or while you're downloading a large file.
If you're hearing some constant, harsh grinding and clicking (louder than what it was in the past), the hard drive could be fixing to die. I'd back up your important data immediately, if that is the case.
There is a large cover on the bottom that you can remove with screws. You could open it and listen closer to see which component produces the sound you speak of.