N-14 Cummins Experts...Please Step Up

Hey Guys,
I have an old (98) Kenworth T600 with a red top Cummins N-14 and I've got a noise problem with the engine. I was hoping someone might be able to help me diagnose it. When the truck is cold and you crank it, there is a noise/tapping sound coming from the upper front drivers side of the engine. After about 3-4 minutes the noise/tapping goes away and it runs quiet and smooth like a sewing machine. It only happens when it is cold. At first I thought that possibly my AC clutch had gone out (haven't used it in a couple of months) because that's very close to where I can hear the noise but it seems like more than that. Then I started thinking bull gear but I have lost bull gears in the past and when a bull gear is about to go or goes (starts thumping), the noise doesn't go away. I just had 2 heads and one injector replaced on this engine. Any thoughts would be very helpful. Why does the noise go away after a couple of minutes? What are you guys thinking?
Can't thank you enough for any help on this issue.
68 Goat

 

Heavy D,
Thanks. Your replies are always awesome. The truck has been off since 8:00 AM. I'm going to hear it myself again in just a few minutes. I agree...noises are odd. It seems obvious that the heat is helping the situation. Do you agree that if it was bull gear related, the noise wouldn't go away? In my experience, once that gear gets worn out or loses a tooth, the noise is more like a thump. I almost have to rule the injectors out because of the head work that was just completed. I did have to replace one and we replaced it. As far as the exhaust leak, wouldn't that be more passenger side of the engine? Something sticking in the valve train seems like the culprit if I had to guess...but what?
It's really got me confused...let me see if I can take a short video and post it with my cell. I've never tried to do that before but I will try.
Thanks Again

 

Stranger, I see where you are going with that but I'm pretty sure that's not it. I'll take a look when we go crank it in a while.]
Thanks

 

Heavy, I got the video shot. I did before and after videos...I will work on getting them uploaded. After about 45 seconds the knock goes away. The best way to describe the noise is a marble being kicked around the #1 head. The knock is totally random and higher pitched like a marble or rock. Weird. Let me work on getting the video posted and I will get back to you.
68 Goat

 

10-4 ..John, am I correct in calling it a jack shaft in an N-14?