I am about 20 hours into Phase 1 flying and I am getting a vibration that I can't find a source for. During climbout (or at level off) with the power set at about 2500 rpm I feel/sense a vibration that seems to be coming from the right side cowl area. There is no vibration in any of the controls but I have found it subsides somewhat if I add enough right rudder to move the ball out about a quarter of a ball width to the left. I have noted the vibration since the first flight and have checked all the control surfaces, the cowl, spinner, etc. and haven't noted anything out the ordinary. I have checked the clearance around the exhaust pipe (there is plenty) and the lower cowl around the exhaust is secured with screws into plate nuts. Airspeed does not seem to be a factor although I have not checked it over a wide range - just 120 to 140. I will try other speeds on the next flight. When I pull the power back to 2300 or so it goes away and when I at max rpm I don't notice it. Any ideas would be appreciated.