I have a 85 f250 6.9 diesel, has 235000 miles. Within the last month I have replaced the glow plugs, relay, and selinoid because it was getting harder to start and i wasn't hearing the buzzing noise when the glow plugs were cycling. It has a manual glow plug switch that was on it when i bought it. I tested to make sure power was getting to the glow plugs, and each glow plug was getting 10V. It starts fine when its warm and it idles fine but won't start without being plugged in if temp is below 30. After doing all those repairs I still don't hear the buzzing noise when the glow plugs are cycling, what makes that buzzing noise? Is it the injection pump? Any help would be greatly appreciated.