Look for one of the crankshaft welch plugs in the pan, you may be losing oil pressure up in that cavity. Sounds like what happened to mine too, no bang, just a loss of oil pressure when I cranked it one day. Under load, it would still show 20 lbs, but at idle the gauge went down to near nothing. I had it rebuilt with a different crankshaft, rings, new bearings. Now under load it shows 50 lbs of pressure, and at idle about 20 lbs of pressure.
Since being rebuilt the first time, that engine had had a "hard to turn it over when hot" issue. I had replaced starters, cables, checked all connections, no help. While it was apart fixing the crankshaft, we noticed some piston scuffing at the top of the pistons on one bank that was (I believe) the cause of the hard to turn it over when hot problem. The machinist said something on that bank had been installed wrong, I wish I could remember exactly what he said. Anyway, he fixed that and honed the cylinders. Now it turns over when hot no problem, cranks every time.
I have that used crankshaft for sale, minus a welch plug, if anyone wants it, and another 86H crankshaft that has been built up and reground to standard size, never used. Send a message if you are interested.