1998 Buick Century Noise while driving

My Father-in-law has a 1998 Buick Century that he says makes a clunking noise when he drives down the road. But it does not do all the time just every once in a while but when it starts it will do for quite a while. He thinks its the cv Axle I told him I've never heard of a axle making that noise and advised him not to spend that kind of money unless your possitive thats the problem. What do you think?