hey guys i built this awsome custom box with the rigth air space and everythign and i have 2 12 inch mtx 4000 subs hooked up to a profile 600msx amp. i am running about 220 watts into each speaker. when u ar ein my car they slam real hard but not to best quality i am guess cause my subs were 45 bucks a peice new lol. but anyways when i am outside my car with the bass all the way up and the volume loud all u hear is my car rattleing like u can't belive. its horrible!! and u don't hear any bass just rattleing. my freinds got the same subs in his civic and they don't hit as hard as mine due to hius amp but the bass outside his car is amazing. oh and my other freind has 3 10inch jl's runnin off one kenwood amp. jl's are the best subs i have ever heard. but i don't understand how he ran 3 subs into his amp. woudln't that only be 1 ohm

 

looks like its time to get out the wrenches, ratchets, screwdrivers, and expanding foam and to get to work. try to isolate any of the rattles and fix em however you can. tighten up bolts, put the foam between panels to stop them from touching, etc. it may take awhile, but you wont sound like every other idiot out there who thinks their system is awesome because they've got a couple subs . its actually kinda a blessing to have your car quieter outside; cops are less likely to give ya a warning or citation for noise. if you wanna sound louder, open a window or a door. your friend's car is probably louder because it is not quiet as insulated/deadened. but please take care of those rattles. there is nothin more annoying than someone blasting rattles down the road.....

 

Well I dont know about anyone else, but I think that the Bass is what pisses people off the most, not the rattles.
But, if you want to fix the rattle problem, buy some Roadkill, or Dynamat. Installation is self explanitory. When you put that stuff in it will sound 100 times better inside, and thats where it counts.
Then if you are not satisifed with the sound outside, buy some better equip. I'm not bashing, but; you pay for what you get.

 

being someone who is very into car audio, a car blasting the bass and rattling down the road is not impressive at all. its almost as if they did a half-*ssed job. but, thats just me. the more you learn, the pickier you get (and its happenin to me....)