Strange popping noise comming from subs?

[TwinkeyMerchant]

My cousin just refoamed his jl w3 12s. He has a jl 500/1 running to them and since he just refoamed them i advised he turned everything on the amp and his radio controls down. Now his subs are making these strange popping noise.. someone in our highschool says it sounds like the voice coils are hitting something.. im guessing the motor? i read a lot almost everyday on the forum and you guys have never let me down. if you guys could give me suggestions on what may be causing his subs to make this popping noise it would be appreciated. Oh yeah, and the quicker the better as he is very.. very stupid, i fear he might could blow these subs doing this and he don't care if they make the noise or not.. when im riding with him he will still play these almost full volume even when they sound like shit.. even with as much warning i give him he never listens.. like i said very very stupid.