Unknown 7.3l IDI knocking
 

I have recently acquired a 1991 f350 with a nonturbo 7.3 IDI (174,000mi) with a zf5 five speed, this is my first diesel truck and it is bone stock. It is hard starting all the time, even when it is hot. On startup i get some smoke, then it clears up after a few seconds. I have done basic maintaince and replaced the glow plugs. But there is a very distinct knocking sound coming from the engine, it happens all the time hot or cold and at all rpm levels(thought it's hard to hear at high rpms). I have used a stethoscope to track the sound to the rear left of the engine. I thought it was a collapsed lifter so i pulled the valve cover to check, and all the rocker arms are solid. Next, I thought it could be an injector so I cracked each of them to see it any of them dulled the knocking sound, and they didn't, the truck was just pulled down by the miss, but the knocking continued. My last thought was the clutch on my zf5 transmission, but while I hear the knocking in the bellhousing it appears to be loudest on the engine(cylinder 4 I believe). I am stumped on what this knocking is, by the location of the knock I dont think it is rod knock, I hope. Sorry for the long post. Any help is appreciated. Thanks in advance.