I have an o4 that does the same thing. aparently the noise will not do any harm, although it drives me nuts. I am going to replace both of mine myself. I did the research and talked to some techs at my ford dealer. I am a bmw tech, and am familiar with doing engine work. I have discovered that the procedure is not that difficult. It just requires one special tool. It is called a timing wedge. I believe that the phaser kit costs about $180 each. It is ok if you just replace one. although, if you have the tool, it is probably better to replace both. You will also need valve cover gaskets.