As Phil said what you done anything recently.
What type of blades are they?
Going back more years than I care to remember when we had the old style blades with the long tip caps fitted rather than the newer extended spar blades with short tip caps when this happened we traced it to a tip cap change and this was causing the blade to be out of Chordwise balance. Most noticeable on the ground with the Primaries off as a bounce and the track was visibly out. We put the old tip cap back on and the problem went away. I would have thought by now all the old blades would be out of circulation but have you changed a blade recently?