My truck was miking this terrible rattle occasionally, sounding kinda like a bad pcv. I replaced it. Still doing it. Gotten worse and is now fairly constant. Its coming from either the back most cylinder area or distributor. What I've done so far:
Pcv, then pulled it to verify new one wasn't bad. Ran it with line plugged.
Pulled cap, nothing seemed terribly off in the distributor, normal minimal slop.
Pulled the valve covers, all seemed tight. Ran it with no covers to see anything, nothing out of the normal appearing.

It almost sounds like its coming from the distributor, valley area, valley pan. I'm thinking lifter? BUT, then I would expect to see some looseness of the rockers/pushrods and I'm not hearing a typical lifter tick, more of a rattling pcv valve sound.
Are these roller lifters? Maybe a bearing in a roller going bad? I'm not sure what else.

 

Timing chain

 

I don't think so, my other 3.9 didn't sound like this when it needed a chain, and it really seems to sound like its coming from the rear of the motor, definitely not near the accessory drive. I even tried using a "stethoscope" and it wasn't coming from the front, but I couldn't precisely locate it either.
Here's a vid of the sound.

 

Yeah, gets quiet in front, loud from back.

 

so... any ideas anyone?

 

Rear center - loose bolt on the torque converter / flexplate / clutch / flywheel?

Bad distributor drive gear?

Bad oil pump?

(All of those are questions, since I'm not sure which or if any of them could make the sound you recorded.)

Now for the bad ones -

Bad main bearing, bad crank, bad cam bearing, bad cam?

RwP

 

What I can theoretically eliminate is that it did this before I did the clutch, and I just did the clutch. The flywheel and pressure plate bolts are torqued to spec.

Oil pressure is perfect, though that doesn't mean something isn't loose.

I was thinking distributor related, seemed ok when I pulled the cap to check it, no excessive slop. Still doesn't rule that one out.

As for bearings, you can hear in the vid is does come and go given the right throttle and randomly once it warms up. So I'm going to say probably not main, rod, or cam bearings.

Maybe a lifter? Are they rollers? Maybe roller bearings in a lifter?

 

They are rollers - all 3.9 LA motors are rollers.

I dunno, I'd be scheduling that motor for a teardown to find it.

RwP

 

Possibly a bolt missing or loose on the inspection plate between the motor and trans (behind the oil pan), causing the plate to rattle? Try listening from underneath the motor.