The check engine light came on on my civic, however, I am noticing a knocking sound in the gears when I accelerate moderately after shifting my automatic into D3 or when the car shifts itself into 2nd...

If I accelerate from a standstill VERY slowly, I wont get the knocking sound, but if I accelerate normally I will... also if I am accelerating while the car shifts, I get the KNOCK KNOCK KNOCK sound...

This seems to me like it is a transmission problem but I have no idea really... can someone tell me what they think it might be? Thanks