I feel so stupid, haha. Turns out I didn't oil enough of the "rod joints" where the rods connect to the wheels. I kept thinking the sound was coming from the front left wheel.
I did what harmonyards suggested and pulled out the worm gear. I held the engine next to my ear and kept rolling the wheels until I pinpointed exactly where the sound was coming from. Hit the spot directly with with an oil needle (before just dropped from further away).
Not sure why the sound started, but will keep and ear out in case it starts up again.
Sound is gone. Thanks all!