Been trying to identify a noise I've been hearing coming from my 03 Blazer. The noise happens when the truck first takes off after coming to a stop. It's like a rattle/clanging noise that eventually goes away once the truck gets up to speed. At first I thought I had screwed up something in the differential, since I had pulled the axles to replace brake backing plates. But today I pulled the cover off the diff and everything was okay. I then was able to duplicate the noise by rocking the truck back and forth while in park. I found that the noise is coming from the front of the driveshaft, near the transfer case. I was thinking u joints, but there doesn't seem to be any slack when I turn the driveshaft by hand. I have a couple videos to show what's going on.

https://imgur.com/a/l142phC

https://imgur.com/byCQXcI

I can't get the videos to embed for some reason so you'll have to click the link.

Any ideas what this is? Could it be from inside the transfer case? Or should I try to change the u joints?

 

It sure sounds like a u-joint to me. Maybe wrap something around the front u-joint (duct tape, old socks, whatever) and see if the noise changes when you rock it back and forth?

 

Since this happened immediately after a repair I would personally suspect something I did or didn't do during the repair...if nothing else is evident. I would open the diff back up and look for metal on the magnet. Plus it might help you further narrow in on the exact part that is clunking. When you removed the the axles did you take anything more apart in the diff besides just removing the pinion shaft lock bolt, pinion shaft and c-clips? When you put it back together did you insure that the spider gears on both the top and bottom were correctly lined up for the pinion shaft? I'm assuming you have the G80 Diff, I can't quite tell from the video.

EDIT: One other point, did you insure the opening of the c-clips were in line with the opening of the block before you pulled the axles back into position?

 

I'm almost positive it's not coming from the differential. I did open it back up yesterday and there was nothing on the magnet. I rocked the truck back and forth with the diff cover off and I'm positive the sound wasn't coming from in there. The more I think about it, I remembered that the whole reason why I replaced the brake backing plates was because I was hearing a rattling sound. I thought it was the remenants of the backing plates that was making the sound. I was probably wrong, and it was this noise the whole time. I think I may just take a chance and try replacing the u joints. Their the originals and they're relatively inexpensive. I think I'm also feeling some vibration that could be a sign of bad u joints. Anyone have a good how to on u joint replacement? I tried looking around hear but couldn't find anything.

 

Replaced both U joints, and the noise seems to be gone. Drove several miles on both highway/back roads and didn't hear anything. No noise when I rock the truck back and forth either. So I'm 90 percent sure it was a bad u joint. Have to drive it more to be satisfied the noise is gone. Also while I was under there I checked the fluid in the transfer case and it was a little low. Not sure where it went, haven't seen any leaks. Is it possible the low fluid level was the cause of the noise?