Possible noise in rear, 2012 Acadia

[Briannafaith97]

Hello, just purchased a 2012 Acadia with 102k miles. Recently after driving, we some times hear this god awful noise, some times at idle, and sometimes in reverse or in drive. It almost sounds like a horrible trumpet noise or if a car rode by with a chopped exhaust. It sounds like it is coming from the rear and can be clearly heard inside the vehicle. We are totally stumped. It happens at complete random times and some days it doesn’t do it at all. Has anyone ever heard of this or may have any ideas as to what it could be?