My dad is having a problem with string noise on his Trykon XL. Here's the details.

2006 Hoyt Trykon XL 50-60 pounds (set at 58) w/ a 28" draw. He's getting a very loud vibration from the string on every shot. He has the factory string (waxed) w/ the factory string silencers. I checked his accessories (whisker biscuit and HHA sight) and everything is tight. The noise is definitely coming from the string. After the shot you can watch his string vibrate like a guitar string for a few seconds. He's very frustrated. The bow has been loud since he bought it. Any suggestions? Thanks.