BUENOS AIRES (Reuters) - An abnormal sound detected in the South Atlantic ocean around the time that an Argentine navy submarine sent its last signal last week was  consistent with an explosion,  a navy spokesman said on Thursday.  Spokesman Enrique Balbi described the blast in the morning of Nov. 15 as  abnormal, singular, short, violent  and  non-nuclear.  A huge sea and air hunt is being conducted for the ARA San Juan, which went missing with 44 crew on board. 