Loud knock on start-up

[1983 Old Blue]

1983 Mercedes 240 D sometimes starts with a large bang. After starting the engine runs perfectly fine with plenty of power and very little diesel noise. I thought the problem to be glow plug related because it eliminated if I cycle the glow plugs two or three times. The glow plugs checked out ok but I replaced them anyway. I used a reamer to remove carbon but found very little. The glow plug relay checked out ok as well. I replaced it anyway. On startup each glow plug has 10 volts on them which should be enough to do the job. I am out of ideas. Help