aflack haha, Buellysses is prolly right, or it could be any number of things loose. prolly have to go thru a process of deduction.
see if you can hear it without doing the wheelie. See if it makes the sound when you land a stoppie, if so its in the back., I'de also do a fastener check. don't forget the rear shock mounts. could be something dumb like that. Could be the forks hitting the stops, could be the steering bearings, maybe someone else may have a clue.