Delrins knocking?

Does anyone know if the Delrin bushings can 'move' and cause a knock?! Under braking as the cars weight is transfered, I can almost feel through the frame, and hear a knock / clunk; I've double checked all my allignments (toe did need resetting) but knock still there, could it be the Delrins moving??

Also, will spherical's (...way to go) hold up to much road use, if so will be top of the Christmas list!!