Just listened to the video and I kind of agree with a couple of the guys in the other thread that it could be the exhaust flap doing it's thing. If it's something internal, I'd imagine it would be doing it at least somewhat continuously, not just a quick knock and then be gone until the next time you hit the gas.
However, I also wouldn't pay anything before it's fixed. Not even a refundable deposit. Not sure where you are and whether you need one immediately or not, but there's another one without any issues around the corner somewhere. Just need to be patient.