DC and the big boys use a padded felt to go on the cones to prevent lead slap. but yeah it could be tinsel slap, spider coming loose, magnet loose from the basket or any thing. you just gotta go over the whole dam thing to find out what it is. but those subs dont cost a lot to replace. if its in the trunk and you cant hear it make the popping noise bang that bitch till it blows lol