I had a similar noise about a year ago when accelerating between 1300rmp-2000rpm or so. It sounded like a howl with a metallic buzz mixed in. The exhaust was stock so I took it to the dealership. They said there was a loose baffle in one of the mufflers, which they said has been happening and replaced the exhaust under warranty. It had been that way since I bought the vehicle new in 2009, just thought it was normal until it got louder and louder after 15000 miles or so. Not sure if that is your problem but it might be worth cheking out.