I hate to do this to y'all, but I need some help diagnosing a rattling noise I've had for some time. I've attempted a forum search as well as many google searches to try and find a write up with similar symptoms. I haven't been able to find a perfect match, but after I've explained my noise scenario I'd be very appreciative if anyone could link me to anything I've missed.

1995 Pickup, 22re 4x4 with a W series trans.

I would describe the noise as a dull metallic rattle, like something metal bouncing around in a can. It is not perfectly consistent, which is why I would call it a rattle and not a click. I've tried a few times to record the noise, but this is challenging as it can only be heard with the wheels are moving. On the recording the engine/road/wind noise drown out the sound, but I can hear it very well in person.

I cannot hear the noise sitting at idle, or when I rev the engine in neutral. The noise starts when I accelerate past 15ish mph, and sounds like it is linked wheel speed (it gets faster as the truck goes faster). It gets slower as the truck slows down, the disappears as I go below 10mph. It also sounds like it's coming from underneath the cab, maybe toward the passenger side. Gear shifting and clutch do not affect the noise at all. It is present in every gear. Also, if I am accelerating while listening to the rattle, then shift into neutral and coast and the noise is unchanged. Obviously, I can hear it better when the engine quiets down and returns to idle, but the noise itself seems to remain unaffected. I can hear it very well while driving next to a wall or line of cars that the sound can bounce off of.

A few days ago, a friend and I put the truck up on some stands and simulated driving to hopefully pinpoint the noise. We started in 2wd, accelerated all the way to about 50mph, and we could not hear the rattle at all. In 4wd however, the noise was evidently there. Tried with hubs locked and unlocked, heard the noise both times. We repeatedly switched between 4wd and 2wd, and continued to have the same results (noise in 4, no noise in 2). Also, while standing right next to the vehicle while performing these tests, it seems as though the noise may be coming from the transfer case area.

One more piece of info, I was crawling around under the truck inspecting and shaking things around when I noticed that the passenger side gear shaft on the front diff has quite a lot of play in it. I can easily wiggle it side to side, up and down and in and out (not as much in and out, still some though). I don't know how much is normal, but to me this feels like too much. I suppose the noise could be coming from this area, but standing next to the truck, it sounds like it's originating further back (toward the t case).

All of my daily driving is down in 2wd with the hubs free, so I don't understand why I only heard the noise in 4wd when I simulated driving with truck on stands. Especially considering that the noise seems to be coming from the transfer case.

I'm preparing to remove the t case and front diff soon, but if anyone has any info that might help me out, I'd really appreciate it.

 

remove your front driveshaft and repeat the test. if the noise is still there in 4wd it can't be coming from the front diff or axles or front driveshaft, it HAS to be in the T-case.

 

Genius! I will do this tomorrow and report back, thank you

 

Promise I didn't forget about this thread. I struggled a bit getting the front driveshaft removed, but after a good pb soak I got it off.

Lifted the truck up and did the same test, accelerated through gears up to about 40 mph in both 2wd and 4wd and I heard no noise. So I lowered the truck and drove it around with the front driveshaft removed. I drove up to 50ish mph in 2wd with the hubs free and locked, then I switched to 4wd and drove around. I couldn't hear the noise at all. Even driving by stopped cars or a wall or something, I could not hear the noise I was hearing earlier.

I'm thinking I'll pull out the front diff, because I want to fix that shaft wobble.

 

Check your driveshaft REALLY well, make sure you don't have any play in the double cardan joint between the two u-joints on one end.
if the centering joint inside wears out or fails it lets that whole end of the driveshaft wobble. and it's right at the T-case.