The first sound, like a thunderous rumble, was a bona fide explosion. The second, occurring right at their doorstep, was shriller and more distinct. It wasn't produced by something shattering, but rather by a series of piercing shrieks that overlapped within a short period, culminating in a booming that only comes when "something is smashed all at once".
The sound was incredibly similar to the screaming whistle of a cannonball displayed by the Titans in the suburban training ground just before it hit the ground. The only difference was that the loud noise came from beneath. After the thunderous noise, everything fell silent.
