Why is my 1997 Plymouth Neon is making a clicking noise when I drive?

I noticed while I was driving in town and the window was down that my car seemed to be making a clicking noise. The faster I go the louder it gets and the faster it gets. What could be causing this? A friend suggested that I might need to add some oil because it sounds like a valve. Im concerned. Its a 1997 with about 77,000 miles.