Could be a bent rod also could be a rounded out lobe on the cam not opening the valve all the way or the timing brake on the head isnt working because of age or the anti knock sensor isnt working. We were just going for the most likely causes. If you have alot of power loss or fluid loss or smoke blowing out the back. But noise is usually due to lack of lubrication hence the tubes. You can take the head off and push out the piston to know for sure or measure from the crank side but the noise wasnt a knock to me it was a valve noise. Good luck in finding the issue.