See now this my good people is why the FTE is such an amazing source of info. Good people. Good intel.

Thank you so very much for your time to write all that. I learned a lot and I'm sure others did too. I'll start poking around to see what I can find.

 

Rod knock I'll put money on it

 

Wouldn't I be able to reproduce a rod knock while revving the engine when parked?

 

Rods rattle when there is no load and "revving" and engine doesn't give it a chance to have the no load, revving has power up load and deceleration load

 

Yes rod knock would likely be more noticeable when stopped due to no other conflicting noises

 

If it is a rod or multiple rods, raise the engine RPM slowly through the range to about half throttle and you will hear it. No load on the engine and it will be apparent.

 

Do you have a PTO? The PTO on my T98 will "rattle" when not under load. This is because of poor shimming to the case causing gear backlash rattle.

 

No PTO. I wish I did!

At lunch I got the engine warmed up to 170. Oil pressure at 40 PSI. I slowly accelerated to 1000 RPMs (to the count of 5). Then let off the accelerator quickly. No rattle. I did it again but this time let off slowly. No rattle. I did the same at 1500 RPMs, 2000, 2500, 3000, 3500 and 4000. I never get above 4K RPMs so no need to go further since the sound, when in motion, occurs below this.

So I crawled under the truck and gave the drivetrain a wiggle. The U-Joint at the rear axle is solid. But up at the front of that same section it does this:

 

What is the condition of the center driveshaft bearing and the rubber that it is mounted in. There should be very little lateral movement in the driveshaft.

 

If I hear what I think I hear in the video, metal on metal, the joint is likely dry and worn out. Hard to hear tho.

 

The center driveshaft bearing appears rock solid. I can't wiggle the front driveshaft from the transmission to the center bearing at all. The only play I have is the front of the rear driveshaft where it meets that center bearing. That's what's in the video.

 

Ok, yep, that "rattle" you hear is those needle bearings rotating around the brinelling they've made in the cross that's been out of lubricant for awhile. Common issue on high mileage "permanently lubricated" universal joints. This could also be a latent issue from a previous owner(s).

 

Okay a new universal joint it is! Obviously we can't say for certain this the source of my rattle noise but I'll replace it since it's bad and see if it goes away. Gentlemen: thank you!

 

If one is bad, the other joints probably are not far behind. Be sure to index the driveshaft parts before disassembly and install them in the same relationship.

 

It is human nature if not hot rodder nature to assume the worst but as I have been a mechanic for most of my life ( and I still do the same ) some times its not as bad as we think hope this is your problem