I'm guessing that he could be a couple of things. Your arrow could be making fletching contact somewhere it should not. It could also be an indicator of bad form, such as not following through after the shot and pulling away. Make sure that you are not torquing the bow grip either. I have had this problem before and my bow was tuned so I knew it must have been me. Your bow could also be out of tune I guess. http://www.eastonarchery.com/downloads.asp

Download that and see if it helps.

Maybe someone more experienced can chime in and give their .02 cents.