Does my loud QX4 need expensive repairs?

My 2001 infiniti qx4 runs very very loud. About a year and a half ago I was told by a dealer that this problem could be fixed by replacing the front converters, but that it was a cosmetic issue that wasn't effecting the longevity of my car. It's over a year later and it's gotten progressively louder, especially on the highway, but seems to run just fine. Is the advice I've gotten true? Is the loud really just cosmetic or do I really need the expensive converter replacement to get my QX4 to last?