is it coming from the motor or the wheels. if its only happening while you are in gear, im assuming that means also while you are moving. the only difference i could see would be that your wheels are rolling making more parts move. you could try putting the car on jackstands and spinning the wheels and seeing if the noise is there. you could also very carefully put the car in gear and see if you hear the noise, just make sure those are some nicely placed jackstands. other than that i would say live with it until you stumble on the cause of it somehow later on.
thats a very clean motor you have there. i say you get the blue wires to finish off the look though. the header hasnt even changed color yet they are so new. good job.