your friend does not know what he's talking about.
a subsonic filter is a high pass filter ment to block out the really really low freqs that a woofer in a ported box cannot play. In a ported box you have a tuning frequency, any freq played full tilt well below that tuning freq will damage the sub. Example, if your tuning freq is at 35hz, then pretty much anything 30hz or lower will damage your sub.
this has absolutly nothing to do with your rattles. rattles are not all made by low freq, they can be made by all freq that the woofer produces. Every car is different. You need to either invest in some sound deader, or mcguyver together some kind of cushioning or bracing directly at the spots that rattle. I recommend foam weather stripping at your local hardware stores. You'll wanna stuff a bunch of this stuff behind the panals to prevent them from rubbing and making noise when the bass vibrates them.
good luck man