+1. If you bought it used either take it to a shop or provide us with a more detailed description. Otherwise we just get a lot of wasted time with guessing games and "this happened to me" stories, and you learn nothing about diagnosing a problem.
Troubleshooting is a logical process, based on knowledge of how things work together, so we need to know things like how often the noise occurs (wheel revolutions, irregular, continuous etc.) under what conditions, in what gears, whether there were any falls, equipment changes, etc. that occurred just before the noise started, etc. and most importantly a description of the noise!
I would suggest you also read
http://www.bikeforums.net/showthread...ease-READ-THIS - the first post and mine near the end.