This from an expert on sonar:

"Unlikely that just banging on the sub's body could be detected,
as the energy decreases like 1 over the distance squared. Some
subs have pingers for such situations".

Of course, it depends on where the sensors where -- but it does appear
they would have to be quite close to the sub, for such banging to be
detected. Just how strong can a wave from such banging be, to travel
more than 2 miles and still be detected? As I wrote, the implosion
is a different story, it's far more powerful.

I asked him about emergency protocols (not sure whether he knows, see
if he returns on that).

Anyway, if they had a pinger, they would certainly have sent something
more complicated than just a single ping every half-hour.

But, some links say that the sub did not have such a device.

Let those who made the banging claims release all the information, esp.
whether it persists after the sub is no more, where was the sensor that
allegedly detected it, how separable are they from random ocean noise, etc.

Note the "roughly haf-hour" reported in some places, by the way...