His description couldn't be used to describe anything scientific. Several of you mentioned turning off ANC making the noise go away but I don't see any reference to you or the other poster who had the same problem verify your liftgate stops were adjusted properly. The terms the OP uses to describe the problem are nearly spot on to hundreds of previous posts of similar cabin buffeting sounds.
My ANC works great...even better than it did on my MXP. Do you think turning off your ANC and having the problem vanish means you've truly fixed the root cause of the problem?