xia... does the new string serving fit the arrow nocks tighter than the old one ? Just curious. Are the arrows holding on tighter and causing their own "plucking" when they release ? I have a bow I am working on right now that was doing the vibration thing and some silencers on the string stopped it. The "thud", to me, means structural concerns or brace height issues while the vibration means string issues. Again, my thoughts only. also, try moving the silencers on the string to different positions, see if that helps.
ChuckC
ChuckC