I have a RV-7A with 65 hrs on it. It has an XP-360 with 8.5:1 pistons and 1 mag and 1 electronic ig. The prop was tested at 0.08 cps and balanced to 0.04 cps at 60hrs. I am getting breakage of the alternate air cable near the air inlet. My airbox is also experiencing breakage (lost 3 rivets). There must be some type of high frequency vibration. I have an MT prop and typically run at 8,500ft at 21mp and 2370rpm. Should I be running at a different rpm? Should i go lower to 2300 rpm or go higher at 2,400rpm. Does anyone know which rpm band is best to operate at. (I vary the rpm typically throughout the flight from 2,350-2,380)