I am an Electronics Engineer with hydrographic experience. I read a report that the acoustics 'pings' where man made but the carrier was not at the expected 38kHz but 32kHz. When I saw that I said, "Woops!" The carriers of deep water sonars are often at 1kHz multiplied by some power of 2. So 8kHz, 16kHz, 32kHz, ... etc. So I considered that the most likely source to be from a surface vessel.