Does this plane have a flat bottom wing? If so, it could be normal. Flat bottom wings tend to want to climb as you go faster. If this is the case, find a throttle setting that you like flying at then use the elevator trim to achieve level flight at that speed. I suspect this is all you are dealing with.

If this turns out not to be a normal characteristic of the plane then look at things like CG, wing incidence, stabilizer incidence, engine thrust, etc. It could also just be out of trim really bad requiring that you adjust the elevator clevis in order to get it right.