I've got a stock (not for long) 91 K5 with 30k miles that sounds like a damn diesel under light load. I backed off the timing 3-4 degrees and it seems to have helped a little. I ran 93 with a booster and that seems to have helped a little too. I also changed the plugs (no diff). The ECM says all is good to go. Any ideas? Does anyone have a source for tech info on 91 engines/TBI/ECM?