I believe the pings are coming from the individual C-probes or their mobile equipment. Something destroyed the stage two lander and it was big. It could be one source of the pings or the pings could be a means of directing it.
The data Al-Q sent suggests there is more than one source. That is why I am speculating the C-probes are the sources. If that is the case, the variations in intensity can be explained as the variations in the proximity of the pinging probes.