I don't know the type at-all, but agree that it sounds like a low amplitude neutrally damped Dutch Roll.
Quite a lot of aeroplanes do this, sometimes even with the yaw damper serviceable (which may have been programmed to only bring the DR characteristics into acceptable limits rather than remove them altogether). It is only a problem if the amplitude is great enough and/or the damping low enough to interfere with good control of the aeroplane.
I'm guessing that the period was probably somewhere around 3-5 seconds?
G