75 Cadillac Eldorado 8,2l (noise engine)

Hello guys, since finally after engine repair my 75s cadillac eldorado 8.2l runs again the first time, after the first run in the stand a kind of “folding” or “”beat”” in the area where the ignition distributor is located or behind the water pump noticed. Does anyone have an idea what that could be? With a short video where you can see it when you listen closely. Greetings Marcel