Popping noise from sub?

I got an alpine type r with an mrp 500 powering it. i have had the sub for two weeks and worked fine. A day ago the sub/box fell over and landed on my amp while i was driving. then a day after it fell my sub started to make a popping noise that starts around medium to low volume. i took the sub out and wired it directly to the amp trying to find solutions and it still pops. i hooked my old type s up and it popped also. i tried my type r in a friends vehicle with a different amp and it worked alright. PLEASE LIST ALL POSSIBLE SOLUTIONS