1972 Chevy engine problems

[72chevylwbmo]

Hey everybody. I bought my son a 72 c-10 back in the summer and have been working on it ever since, not having much luck with it. The other day he told me that it was making a knocking noise. So I drive it around, I don't hear a knocking noise. It sounds more like a really bad octane ping. I can stop the truck and put it in park, rev it up and it sounds fine. Put it back in gear and take off and the noise is back. Pulled one of the valve covers off and the oil smelled burnt, really bad. Going to run some kerosene or something thru it to clean it out, then change the oil and see if it clears up any. The motor is a 350, th400 trans. Already had trans rebuilt. Any ideas as to what the problem could be. I'm not a real good mechanic or at least the diagnostic part. Any help is appreciated.