Question!!!!

So i finally got it all sounding great, but on a select 1 or 2 songs (Example is "Put on" by Young Jeezy, at a particular point of the song, i will hear a large discerning klank of a pop, as if the subwoofer was bottoming out... its happened a good 4 times now, since i had no idea it was about to happen or what the hell it was lol...

But its strange since the amp im using is 600rms @ 2ohms, 400rms @ 4ohms, on a Single 3 ohms 600rms 1200 watt peak 13.5JL....

Do ya'll think it issss the sub bottoming out and i just need to watchout for those songs?? It happened right as the sub was going from a mid sound to a low one in both instances.... Hate to hear this sound evvveerrr again lol...

Thanks