Let me start by saying that I searched this site for 3 hours and I didn't find my exact problem. So I'm creating a new thread.

I have a 4x4 1999 S10 Blazer. Not sure if this matters, but it's a LS.

There is this really awful rattle coming from somewhere underneath the car. Took it to the shop, and my mechanic said that it was coming from the t-case, and that it needed to be replaced (as well as the encoder motor).
So I replaced them both with used parts from my other blazer, but the noise remains. Also installed new universal joints and front and back driveshafts.
I don't know what this noise could possibly be because I know the replacement t-case and encoder motor are good. Any help would be much appreciated.

 

Some more info on the noise would help diagnose it.

Is it speed dependent, as in it gets worse the faster you go? Or is it only at certain speeds over 35mph?

Is it RPM dependent? Is it worse at higher RPMs, or at a certain RPM?

Does it only happen during acceleration or is it constant while moving, no matter speeding up, cruising at a constant speed, or slowing down?

Does it happen worse when going over bumps?

Has there been any driveability issues that you've noticed? Lack of power, hesitation or chattering when taking off, etc?

 

It is speed dependent, but nothing else. It doesn't happen below 35.
Between 35 and 55mph, it seems to be intermittent, while anything above 55mph is almost constant. It's like something is loose underneath, but when I was under it, nothing seemed to be loose.

 

What exactly did you check for looseness? Aside from the obvious drive train possibilities (drive shafts, front axle shafts, etc), make sure you check control arm bushings, ball joints, sway bar links, torsion bar mounts, shock mounts, leaf spring mounts and bushings, muffler joints and hangers, and so on. Any small vibration can turn a loose bracket or joint into an annoying rattling sound.

 

We checked everything we could reach, but nothing seemed loose. Now that I think of it, I don't think I checked to see if it was the e-brake. I hear those come loose quite a bit.
I drove it a little today to see what it would make the rattling noise in neutral, and it did. That means that it is not the drivetrain, correct?

 

Ah, quite possibly. Does it sound like it's coming from under the hood area? How loud is this rattling?

It's common for the PCV valves on these to be quite noisy and make a faint rattling sound. I chased a faint rattle sound down in my 04 that was consistent with RPM's, mostly noticeable at idle actually, before I found the PCV valve to be the culprit. Was easy to check; place your hand on the PCV valve while the engine is running and you can feel it "clicking", which is what makes the rattling sound. Give the engine some RPM's with your other hand and see if the sound changes for better or worse, and if you can feel the clicking correspond with the sound. If it is the PCV valve making the noise, you might notice it being quieter with your hand on it, as your hand muffles the sound out some.

But that's only if the sound you're hearing is rather faint. If it's louder, then you need to start looking elsewhere to pinpoint it. Could be a loose bolt or bracket somewhere on the engine or exhaust still, could be a worn out belt tensioner, ect.

 

Ok I'll take a look and reply in a bit

 

Could be that your muffler or catalytic converter are broken internally. The bast way to check that would be to have somebody listen under the car while somebody else causes it to make the noise.

 

It's very loud, but it's not deafening.

Before I replaced the t-case and encoder motor, my dad and I drove around and determined that the noise only comes from driver's side of the car, and more towards the middle. Besides, the mechanic said that it was most likely the broken chain in the t-case causing the noise, which was ultimately why we replaced it. But the noise stayed, so maybe the new t-case we put in is falling apart as well.

As for the muffler and cat-converter goes, there isn't a cat-converter and the muffler is almost new.

I'm stumped.

 

There is a gear in the transfer case With a selector arm In 2WD nothing is used Once you push the 4WD button That arm sets the gear into the other giving you 4WD If you have a bad vaccum leak / encoder You will have the arm only go in half way Chewing your carrier gear (Differential Carrier Gear Kit Front Dorman 600-561) There is no need to buy a whole diff unless its under $100 And you feel like paying extra on labor Myself i replaced the gear And the actulator under the battery Along with the vacuum actuator switch And checked all my Vacuum lines There is Youtube videos for how to remove and install your carrier kit which is simply removing washers in order The gear Then putting the new ones in. in the same order ..If your feeling mighty...buy a new cable too and clean the mounting area