Periodic pattern in aeroplane contrail

My attention was caught by this contrail. Note the very regular pattern along the far edge. Why is it so regular? Is it due to some periodic motion in the engine? I'd think not guessing that engine frequencies would be too high. And why is it only along one edge? If I see something like this again I need to try to get some idea of the wavelength of the recurring pattern by measuring it against the plane itself.