Okay, I just bought this truck a few weeks ago. Its a 2000 Chevy Silverado 2500 with the 6.0l. Its got a "ticking" or "knocking" sound. I have just about ruled out piston slap and even lifters. It knocks noticeably loud when cold, it gets quieter but definitely does not go away after it warms up. After warm up you can hardly hear it inside the cab unless your really listening. But if you get out and stick your head under the passenger side wheel well its a constant "tick, tick, ticking". It seems to go away after about 1000 RPM's but it could just be getting drown out by engine noise . Also something to add is the a/c has a strange noise when its on. So im wondering if its a bearing or belt or something on the a/c causing the knocking. Either that or maybe an exhaust leak. I didn't really notice the sound until after I got new exhaust. That's why i bought it. Its only got 75,xxx miles so im hoping it will last a lonnng time longer than this. Any help will be appreciated.
Jake

Also just in case it was the lifters I have tried sea foam in the crank case and through vacuum line. I have tried heavier weight oil additives. Nothing seems to help!