i have just installed 4 hifonics hfi12d4 subs into my car, running them in pairs in series/parallel at 4 ohms per pair. they are in a sealed box with 2 chambers, 2 subs in each chamber. i am running them on a power acoustic 525w amp, bridged (until i get another amp) subs are brand new, when i hooked all 4 of them up they have very poor sound and produce a vibrating/ rattle sound... even at low volume. however when i unhook one pair the sound quality drastically improves. but the rattle is still there, tho no where near as bad. ive tried everything i can think of and am still clueless as to what is the cause of this. any help would be greatly apperciated. thank you.