Metalic Ticking sound

[Yukon6.2]

Hi All
Yesterday i was headed to the big city with my signature truck to pick up a trailer load of waste oil and a few other things.On the way in about 30 miles from town we heard a loud metalic noise,my buddy thought a fan belt had let lose,i thought a fender from the trailer had broke loose,a quick look in the mirrors ruled that out,A quick glance at the guages showed vitals were all good.Pulled over and found it was coming from the motor.There was a pull out a couple of miles away so i limped it there and got off the road,
I did some diagnostic once we got off the road,my buddy is a mechanic as well.No huffing from motor with oil cap removed,losened each injector and all cylinders were firing,pulled the new glowplugs on the noisy side of the motor and they all looked fine,Harmonic balancer and belt pully were fine.I had a can of Marvel Mystery oil along so i added half a quart and decided to slowly drive to town.Our best conclusion was it was valve train related.
I kept the RPMs around 2000 and the noise wasn't to bad,after a few miles it seemed to get quieter.First stop in town was 9 holes of golf.
After sitting for a couple of hours i started it and if you didn't know there was a sound earlier you would think it was normal sounding.
I decided to drive home we got some food in case we had to wait for a resuce.Once in a while you could hear the sound faintly slow down a bitand it would go away.Made it home,no load on the trailer.
My thought is it's a lifter gone bad ,mabey the MM oil losened it up a bit??
Anyhow i would be interested in anyones thoughts on what i should tear apart to further diagnose the prob,or if anyone had similer problem.
Motor wasn't missing or leaking anything,no smoke,all normal except the. noise,i put 130 miles on it.
Thanks for the ideas
Thomas

[JohnC]

When you say the noise "slows down" do you mean it does not track RPM?

Could it be the vacuum pump (if you have one)?

Did the noise persist when you opened the injector lines?

[Yukon6.2]

Hi John
i meant when i would hear the noise i would slow down the speed i was driving.
Yes the noise follows the RPM,seems high in the motor ,valve train area.
Noise was still there as the injectors were opened,you could see the motor begin to miss when each injector was opened as well.
Never gave the Vac pump any thought,it's working,shift points havn't changed.
Thanks Thomas

[DmaxMaverick]

Many possibilities.

Check:

Accessories. Any of them can wear out and rattle. Try duplicating the noise with the belt(s) removed.

Torque converter and/or flex plate. A cracked flex plate or loose TC bolts aren't uncommon.

Water pump. Check shaft movement and free rotation.

Fan. Check for cracked or bent blades, or worn out fan clutch. Check the radiator, fan shroud and oil cooler crossover lines for indications of contact. A cracked shroud can cause fan contact, and may only be present at higher fan speeds.

Injector lines. Any line in contact with structural metal can make a loud knocking sound timed with the injection pulse.