84 box style caprice with a newer 99 vortec. Left bank has a knocking sound that comes and leaves depending on accelaration. When i decellarate it began, if i softly step to acell it goes away. this happens when at high speeds. can anyone help?:banghead: sorry for the misunderstanding, its a vortec out of a 99 tahoe.

What is a "newer 99 LT1"?

There was a 4.3L version of the LT1 called an L99 that was a V8, and the LT1 was only used through 97.

To it sounds like it has a rod knock. Is it a deep tone or more clattery?

The L99 was only ever installed in the Caprice. It was '94-'96.

96 is right, the last year of the LT1 was '97, and that was in the Camaro/Firebird.

I'll second the rod bearing theory.

If the thing is still running strong, it could be a wrist pin. They give off a "double knock" and is a little more of a clatter. See if you can isolate it to a single cylinder by removing one spark plug wire at a time and going for a ride. The knock usually lessens when it is not under compression.

Bob

thank you for your input.