You guys are always full of useful advice, So yet another problem. I have a 98 disco, blown head gasket and lower end noise. So i purchased a parts truck with a good running engine. The engine had a lifter noise so i decided to rebuild the entire thing before installing. The machine shop polished the crank and resized 3 rods as they were out of spec. I got everything back from the machine shop, got all my new parts together and assembled the engine. I installed the engine and everything was great for the first 200 miles, ran great no noises at all. Now i notice the exact noise that my original engine had. The noise sound like a dull knock, it is more noticeable when the engine is cold and gets very faint as it gets to operational temperature but you can still hear it. Ever thing was assembled correctly and torqued to spec in sequence. I used conventional 10w-30 with plenty of assembly lube applied to moving parts. I have heard that synthetic can sometimes quiet a knock or noise compared to conventional oil. The engine runs perfect other that that noise, I get sick at the idea of spending as much money as i did on this engine and something still being wrong. Thanks for any advice..

 

With the engine off take a small hammer and bang on the cats, if the guts are falling apart they will rattle and can sound like a faint rod knock when the engine is running.
Also switch to 10w-40, it is better suited to these engines.

 

no noise from the cats, you can hear it the most prevalent on light engine acceleration, the thing i find most odd is this is a completely different engine from my original, and it sounds exactly the same. I am wondering if this is just how these engines sound, this is my first Disco and i have never listened to another run. This engine has new rod and main bearings, lifters, timing chain and gears, rings, plugs, wires, complete gasket and seal set including new head gaskets and new bolts. The machine work was done by a professional machine shop.

 

Well if you banged on the cats and they didnt rattle then I dont know what to tell you other than it may be the oil that you are using.
My engine doent make any noise and it has 179k on it.