@IanW
The recordings of an identifiable noise (in fact two) happening at approximately the time that 370 was expected to have 'ditched' is an extreme coincidence.
As mentioned, the first acoustic transient, if originating from the final Inmarsat ping ring would have been produced approximately 50 minutes after the time of last Inmarsat ping.
The second acoustic transient was much later and would have originated approximately 4.5 hours after the last Inmarsat ping
Both signals received contain only low frequency energy, suggesting a large propagation range, and have rather different characteristics (frequency and time spread).