That's what I'm hearing about every 3-5 minutes right now, off in the distance. I think it has something to do with The Crucible - the final of the recruit training. After this, they can officially call themselves "Marines" if they survive it.
I heard it as I was giving Amelia a bath and putting her to bed, and she asked me, "What that noise, mommy?" I was afraid the noise would scare her, so I sort of fudged on my answer (since any Marine who has been through it will probably disagree with me).
"It's just Marines playing and having fun, Mia".
BOOM! BOOM! BOOM!! In rapid succession, we hear cannon fires on the range. Mission accomplished - Amelia laughs in her bed and says, " 'RINES PLAYING, MOMMA!"