When parked and not under load, when I rev the engine it sounds normal. When at speed, usually 3rd or 4th gear, when I'm really getting on the throttle, I hear a rhythmic hissing sound coming from the exhaust. It's most noticeable when I'm riding close to a wall or bushes on the right side where the sound bounces back at me. Based on the engine speed and rhythm of the hisses, it sounds like it's coming from only 1 of the cylinders. What could be causing this? Remember it only happens when under load.