Rattling noise coming from subwoofer, how to fix?

whats happening is kinda like you said, a limitation. the speaker is "bumping" as hard as it can and causes that noise. a good way to test this is if you can actually see the speaker itself as its making that noise. if it looks like a jackhammer is behind it there you go... this also seems to be the case cause of how small your sub is and like you said how cheap. i got myself a 15 incher with a built in pre-amp at 900 watts, that solved my problem, now i just hear my house rattle.