Pulsating lump in neck, movable and soft. Ultrasound of carotid clear. What is this?

Hard to say. A movable pulsating lump could be a lymph node that is lying on the carotid and you are feeling a transmitted pulse. Try ultrasound over the mass to see if it is fluid or solid.If it is solid it possibly should be removed to make sure it isn't a tumor.
Could be direct . Or indirect pulsation. Should be evaluated for blood flow by ultrasound.