[kowboy63]

I had a 98 Blazer with a 4.3 vortec started doing this at around 100,000 miles, I tried all the above except the catalytic converter, kept on knocking. I put another 100,000 miles on it then sold it in 2007 & last I heard it was still running and still knocking. I would have loved to known what the knocking was but I couldn't justify tearing it down while it was running so good otherwise.