That is another resonator in the system in addition to the ones on the tube. It is for sound suppression, same as the others. With the Stillen intake, it accomplishes nothing. The only reason that hole is there is so it mates up with the resonator in the G35 (so you don't have to take it out), but it still doesn't do anything b/c the Stillen doesn't use a closed box. You don't have to take it out, if you don't want, but I DID remove mine; useless fat.