Get better bars I think it might be just the metal flexing. Do they bend a little when you put all your weight down on them?

edit: stem might be flexy too. I used to have a cheap modolo stem to some flexy aluminum bullhorns that were similar in stiffness to a large rubber band. if i put all my weight down on them they would flex about 1/2 - 3/4 inch. they were noisy. Then again on that bike the headset was garbage too.