Subaru makes a "pop" knocking sound when accelerating, ONLY from first gear, not from 2nd or 3rd.

Basically that's it. When I accelerate from 1st gear, with the gas pedal pressed more then half way down, the car makes a load knock or "pop" sound, for split second, then takes off. it seems to sputter, then be fine. Only happens with first gear. Wondering if its the injectors, like a misfire, or if it's something totally different.