Opinions on knocking sound on I6 250

[slaminum69]

Here is my situation, I have a I6 250, its been rebuilt with all new parts and maybe I have 1000 miles on it, there has been this knocking sound I cant get rid off. I have changed out the lifters, rockers arms and its still there. I did track it down to the 5th cylinder, tightened the valve till it went away but then it begins to misfire, loosen it up and the knocking is back, repeat and same situation. I am having a hard time thinking its the cam, but you never know, today I am going to pull the lifters out on the cylinder again and take a look at the cam to see if there is any distortion. Also I noticed on those two rockers arms there was alot of air bubbles in the oil coming from the push rods. So I am looking for some inputs from the I6 gurus on here, Thanks in advance. Dennis