I know online diagnosis is a crap shoot, but here goes.
I took my 54k mile 79 D-50 on a 3 hour interstate drive pushing it harder than I should (75mph). When I stopped for a snack, I noticed a bad tick that rattles when revved. Thinking cam problem/ valve adjustment I pulled the top cover to find a very loose timing chain. While changing both chains, gears, and guides, I noticed the bolt on the oil pump shaft had backed out and was grinding into the cover. Cover welded, bolt replaced, and all back together hopeful I had solved the issue. No dice. Engine is quiet when cold, but the loud tick and rattle when revved still exists when it warms up. It's hard to tell where the noise is coming from even using a stethoscope, but it seems like
Maybe in the area of the front cover/balance shaft area. I've adjusted the valves several times, and inspected the valve train and jet valves with no change. I've searched but can't find the symptoms of balance shaft bearing failure. Is there a way to check the shaft bearings? I also pulled the plug wires one at a time to isolate maybe a wrist pin or bearing issue there, but the noise is the same regardless. I've thought about deleting the shafts, but since I'm not after performance and prefer all stock I'd rather fix the problem.