Tell me this noise isn't what I'm afraid it is.

[Old Henry]

Some progress! I decided to try pulling the plugs one at a time again and see if I could hear the knock through the very loud noise of the air rushing in and out of the plug holes. It about made me deaf but I could barely perceive the knock except on cylinders 1 and 5 that are both hooked onto the same offset on the crankshaft. That's at least a start to have narrowed it down to one of those two.