tl;dr: Sonar buoys picked up sounds at 30 minute intervals. There is a relatively high probability that these are from the missing vessel, as producing sound by banging on the hull at the hour and half-hour is protocol for rescue signalling from underwater. Sound travels well where radar and other kinds of signalling does not. It's also possible that the vessel is at or near the surface; sounds travel better in the same ocean layer, and the buoys are at the surface.