Probably a loose piece. Have someone rev the engine and you look under the hood for the noise. My engine cover posts vibrate from time to time. Now that we are talking about it I've been wanting to take those off, thanks for posting!
Could be your flex pipe. Mine is fine until I accelerate and then i get almost a buzzing sound that seems like its coming from behind the glove compartment. Put it on my hoist and still hear nothing unless someone presses on the fuel.