X2 that what mine was, it was the heat shield on the exhaust, right before the second O2 sensor and after the cat on the driver side and it would only do it in gear and around 2k rpm. And yes put it in gear/D and rev the engine and see if it does it then. If so have some one help you to figure out exactly where its coming from. Have them set the Park brake, put the tranny in D, step on the gas and brake at the same time and look underneath and see where its coming from. It prob best to have some one you trust help you with this though as your gonna be under it while its running and in gear.