I've been flying my RV8 for almost 400 hours. I have done acro quite a bit in those 400 hours, but never with a passenger. A friend with acro ability and I put on our chutes today and he flew from the front seat. I have done several hundred split ess maneuvers by myself. He pulled up to slow to 80mph and rolled right to inverted and started the down side allowing the speed to build. There was a violent shake and some strange noise from the tail. Scared me....never happened before. We talked about what happened for a minute and then I did the maneuver to the opposite side. The same thing happened. Scared me again. We landed and took a real good look at the airplane....mainly the tail. Everything was tight and normal. Was the tail stalling because of the aft CG?