Main bearings make a fairly heavy thumping/clunking sound when loose, which may get louder under load. Main bearing noise also tends to be more noticable at low speed, under acceleration. From your description, it doesn't sound like rod bearing noise- you can't short out the noise. What happens as you accelerate and decelerate the engine, up to about 1500 RPM, and return to idle? Piston noise tends to be more noticable at low engine speed, but often disappears after temperature increases, so it doesn't sound like piston noise. Also, piston noise is usually a light tapping sound more noticable at light acceleration. The decreasing oil pressure and increasing noise as the engine warms up go with main bearing trouble. A worn cam lobe would give you a loud tappet and shiney metal in the oil, but wouldn't affect the oil pressure. The oil pressure numbers you gave seem low, but I don't remember specifically what what is normal. Cold engine should have something higher than 45 PSI, I think, although that might be all right with the engine warm at idle speed.
My guess is that you have a disassembly job in your near future. If you find a bearing failure or worn cam lobe, flushing the engine oil circuit probably is a good idea.
Harvester
