I would crawl underneath to both footwells and push with your hands underneath anything there to see if anything would creak. You can remove the radio and glove box to see anything there would do the same thing. I wouldn't think motor mounts either.
It sounds more like a metal on metal or bushing thing, can also be heard from outside as well(as long as I'm at low revs and my exhaust isn't drowning it), doesn't seem to creak when the clutch is in/neutral or even downshifting. Only when clutch engaged+application of throttle. Which sorta means it is triggered by torquing the motor in one direction.I would crawl underneath to both footwells and push with your hands underneath anything there to see if anything would creak. You can remove the radio and glove box to see anything there would do the same thing. I wouldn't think motor mounts either.