i have two JL subs in my car, i dont know anything about audio, does one of them rattling really bad mean that its blown?

 

Its possible. Here are a couple of tips to see
1. Check your gain levels on your amp. If the gain is set too high your subs can blow
2. Are you still getting sound from it?
3. Do you smell something burning? If you do it could be your voice coils burning
4. Try to gently push down on the subwoofer. If as you push it down you feel it scraping on something or its rough to push in, the sub is blown. The feeling is very distinct.
5. Is the sub flexing at all?

Give some more details as to what it does to help diagnose it

 

it has a wire grill over it, so i cant push down on it, its like it just overly vibrates, and the mounting screws are not loose or anything, its the first thing i checked

 

Your sub may be blown if you see it smoking a cig.

 

remove sub
take an multi meter set it on ohm
place the + & - wires of the meter to the + & - terminal of sub
if you get a reading it works if it does not it is blown