Okay!! I'm not the only one with a 93 that makes that noise! I bet I know exactly what youre talking about. Mine does it when you rev to 2800 - 3000 rpm and makes the noise when you let off to shift. Just for a second or two. If I romp on it all the way to 5k+ or stay under 2500 it won' t do it. In my car it sounds very knocky/lifter, like everything inside the business part of the engine just went temporarily loose. If this sounds about right, I don't have a clue what it is. My engine has 168k on it, but was rebuilt under warranty at 34k. Frustrating. I will never know personally, cause I'm replacing that sound with the sound of two turbos and a couple blow off valves. Let me know when you find out though!!