I would suggest trying to modify the snubber across your synchronous rectifier fet. R109 is too large and C106 is too small. For a starting suggestion, try 10 ohms for R109 and 470p for C106. You would balance between damping ringing at that location without incurring excessive power dissipation in the snubber. It would be a good idea to monitor the voltage across the sync fet during operation to get an idea of the amount of spurious voltage overshoot and ringing.