My brother and I just rebuilt our first engine. It is a 350 bored .030 over...we did a cam break in and everything was fine. Fired it up a few times later and we hear this knock coming from the passenger side. Hard to tell if it is coming from the top or bottom of the engine but one thing we did find interesting was if you put your or any object behind the passenger header (open headers right now) the sound will dampen. Tells me the sound is coming out of the exhaust but what could be causing this sound? Detonation? Rod knock?

Done a lot of reading but this is our first engine we’ve actually put together so any advice is greatly appreciated!

Thanks,
Kyle