Gen 2.5 grinding noise, HELP

So what happened:
I was driving home, and it was roughly an hour trip. I held a steady 75mph most of the way. She's running like butter, solid as a rock (recently paved freeway helps, too). No wobbles, vibrations, or anything like that. Temp is holding solid and oil pressure looks as it should.
Last 10 miles or so, on a stretch where the bypass turns back into a road, I notice a metallic grinding noise. It builds in volume, then drops off. 30 seconds later it builds up again, then drops off with a *shink!* sound. As I come to a red light, it comes back as a rattling grind now, and it sounds like it's coming from the transfer case, even though I'm sitting still. Taking off from the light, I rest my hand on the t-case shifter- no vibration. No vibration from the car, either. None during all of this.
I engage 4x4, and it does so without a hitch. Lights on the dash do as they should, too. I disengage 4x4, and lights go off as they should. All seems well with that.
Acceleration seemed fine, as did braking. I drove the last few blocks with the window down and didn't hear a thing. Got home, parked in the driveway. Revved the engine gently and it sounds as smooth as it had all day. Shut it off and look under the truck- no puddles, drips, bare metal, or anything dangling (except a random stick near the trailing arm).
It's not my daily, so it's been sitting all day today. But I've been going nuts on how to figure out of what I heard was something deeply serious or not. I don't have the money for professional diagnostics right now, so anything I can try, please suggest.
My thoughts are torque converter, but I've had no slipping lately and no shuddering, either. At least none that I've noticed. The mud tires I'm running cause a certain level of extra vibration at high speeds, but even last night that was minimal.

All help is appreciated!!
 

DaveInDenver

Middle Income Semi-Redneck
If it was making noise when you're sitting still but the engine running it is unlikely to be the transfer case, wheel bearings, brake related (like rock stuck in the caliper), differentials, driveshaft u-joints.

Could be something on the input side of the transmission, like a torque converter as you say. Engine, perhaps. I'd suspect something external on the engine if the oil pressure light didn't illuminate.

I'd do the easy stuff first. Check the accessory drive belt(s), spin the alternator, water pump seeing if the bearings are toast. Check the power steering pump and A/C compressor. I'd probably drain major fluids and look for evidence of an issue like metal shavings.

Good luck!
 
Update- Checked fluid levels and all are good, clean, and smell as they should. Trans fluid was shockingly clear; I changed it about 2 years ago now.
Brake rotors look good, no gouging from anything there. Belts all tight and in place. No missing bolts from the engine or surrounding areas.
It sounded like it was under the car, as the sound filled the cabin and by the third time it showed up it was scary loud (though I also had no music or anything so I could hear it better)

I have noticed a squeak in my ac compressor that goes away when I shut it off, but would the compressor failing cause such a racket?
Next I'm going to start it up but I'm just afraid to make anything worse.

And I had tested the tcase just to eliminate it as any possibility. Because all this started around 70mph on down to a stop, I'm checking everything that moves for my own peace of mind.
 
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Second update- After all tests I could think of that I could do in my driveway, the only "result" is a metallic rattle at about 2500rpm, in Park. The moment it drops below 2500 it goes away. Bring it back up to 2500 and it comes back.
Problem is I'm alone so I'll have to wait until my husband gets home so I can look underneath while he revs it up. It sounds like both a vibration of something loose and something rotating, so I have no idea if it's inside or outside, just that 2500rpm is the sweet spot. And it's not the same noise I heard before, but it's the only thing I could get it to do without driving it.

Thoughts, anyone?
 

DaveInDenver

Middle Income Semi-Redneck
Something else that just occurred to me, check for loose bolts or gravel and debris stuck on or behind skid plates and heat shields.
 
*UPDATE* Not a heat shield, but a piece I can't name because it's just out of my limited knowledge. It's at the back of the transfer case, right before the rear driveshaft. It's a concave piece of metal that surrounds the output shaft before the knuckle of the driveshaft, looks like it's supposed to be pressed into place, but has rattled completely loose. It had moved up toward the knuckle but I was able to push it way back into the housing, so I guess that's where it's supposed to sit.
Based on what little I know, can I just hammer this thing back into place? Is there something else I need to look for?
Struggling to find a diagram so I can put a name to the piece. Anyone know what it is?
 
Not at the moment. I can get one tomorrow, though. Already changed out of the crawl-under-the-truck clothes lol
I'll get back under it after work tomorrow. It seemed like it's maybe supposed to protect the output shaft seal on the tcase? I didn't see any moisture, but now I'm worried about the seal and the fluid level in the tcase, so I'll be checking that.
 
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Toasty

Looking for that thing i just had in my hand...
Sounds like the transfer case damper, on the older trucks there all metal and they break. Later ones have a rubber isolated metal block, they're just for driveline noise reduction and you can take it off and drive without it to test.
 
Sounds like the transfer case damper, on the older trucks there all metal and they break. Later ones have a rubber isolated metal block, they're just for driveline noise reduction and you can take it off and drive without it to test.
So, this piece? This is what's rattling. Just disconnect the driveshaft to remove it?
 

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Toasty

Looking for that thing i just had in my hand...
So, this piece? This is what's rattling. Just disconnect the driveshaft to remove it?

That's the dust shield, you might be able to push it back in place without disconnecting anything otherwise yes the driveshaft needs to be removed to remove it.
 

DaveInDenver

Middle Income Semi-Redneck
You Mitsubishi guys please correct me but those shields are usually protecting slip yokes in my experience. So while not absolutely critical they do serve an actual protective function.

If it's similar to Toyotas (who uses these on 2WD transmissions) you could unbolt the driveshaft and pull the slip yoke apart to get at it. If that's the case be very careful to mark the clocking so you reassemble the slip yoke the same way and that splines mate the same. Also it's likely some gear oil will dribble out.
 

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