This could also be caused by incorrect aileron/rudder trim. If you have both ailerons and rudder, you may have trimmed the wrong one to correct a minor left or right turn. Try retrimming by slightly trimming the ailerons in one direction and compensating by trimming the rudder in the opposite direction. This miss trimming is a common error on many planes and goes unrecognized because, in normal flight, it is not evident.