It says the Youtube link is invalid.
But if it is more of a buzzy-rattle, that sounds like something "light" rather than a heavy knock, it could well be the heat shield down the back of the engine, between the manifold/exhaust and the firewall. I seem to remember that something runs very close to that - and caused a buzz/vibration on over-run on mine, just as you describe.
It's really soft/flexible, so just bend it out the way a few mm from anything that it could be rattling against. This was years ago and it never reoccurred, so don't remember the details very well. Just put your hand/arm down there and have a tap around.
If it's a heavy knock, then the prognosis might be worse..!