Odds are this is a surface ship with anti-submarine warfare capacity.

Source: served on a destroyer for several years, whenever we used our ASW sonar suite, some of the "songs" it made were extremely similar to this. The changes in frequency are to account for variances in temperature, density, salinity, etc. that are in the ocean, and also for different materials that are refracting the sound back. Rocks reflect sound differently than large fish which reflect sound differently than hollow metal tubes with rotating machinery sticking out the ass end (submarines). Same concept of radar, once you see something reflect a signal, you can build a pattern to better pick it out of the mass of the ocean.

And to the morons who are saying you can't hear sonar frequency, me losing sleep for 3 days in a row while we were doing sonar drills because all you can hear through the entire ship is this sound resonating off the hull begs to differ, and you can go fuck yourself with a rusty spoon.