Teaser for the regulars

Recently fixed a problem that was slightly strange. Let’s see if you can ID the problem before I did.

2005 Trailblazer, 4x4, 4.2L I6, ~150k miles, white with black tires :wink:
Bought used a year ago.
Have changed; front struts, rear shocks, front sway bar end link

Two weeks ago, it developed a sound like sheetmetal dragging on the ground.
Pulled over, inspected, nothing dragging.
Thought perhaps a heat shield came loose and was hitting the driveshaft or something.
Nothing apparently imminent or dangerous, kept going.
Sound seems to be intermittant and not related to steering direction.

Along the way, noticed it seemed to coincide with acceleration (load).
Shift to neutral and rev engine, no sound.
Trans in drive and light gas to maintain speed, no noise.

Jack up front end to check and find the sway bar end link I had replaced has loosened up considerably.
Strange, but I R&R the lock nuts and use loctite this time just to be sure.
Nothing else appears out of place.

Test run and no more sound. Fixed?

Nope, it gets progressively worse over the next two outings.
Sound has changed from sheet metal dragging to a sharp tinging sound and is really pronounced on acceleration.
Now it is happening all the time and getting more pronounced.
The truck is retired until the problem is found.

Have you checked the “rock shield” (gravel shield) that covers the torque converter? If it gets slightly bent inward…all kinds of noises will be heard.

I’m thinking the motor twists in its mounts a little on accel and that’s what makes “it” happen.
Something shifts and touches what it shouldn’t.

A long shot, but check for debris between the brake rotor and the shield. This happened to my 2000 Blazer, there was a pebble between the dust shield and the rotor. There was an indentation in the shield that was keeping the pebble in place. It was making a horrendous screeching noise at all speed. It took several attempts on my part to find the pebble.

Ed B.

OK, ready for the next clue?

Figure I’ll see if I can recreate the sound in the driveway making it easier to isolate. Jack up rear of truck until both wheels off the ground, stabilize with stands. Shift from park to drive and PING! Hmmm, interesting development…

So I repeatedly shift back and forth trying to repeat the sound and locate the area it is coming from. Hollow sounding ping! that happens occasionally when shifting from P->D, D->R but not all the time. Sounds like it is coming from the rear end. Only doing it maybe 1 out of ten times.

Enlist helper to do the shifting while I crawl under to have look-see. Place hand on rear differential cover and when the ping happens, it can easily be felt in the housing.

Shut off truck and for grins, check diff fill level. Proper fill level and oil looks perfect.

Try rotating one tire abruptly by hand. Back and forth jerking the tire and similar result. Once in a great while, PING! It doesn’t seem to matter what position the tire is in, it can happen at any rotational position.

We’re getting closer!

The needle bearings in your universal joints have turned to dust (rust).

Okay. What about the “C” clip for the axle? It may have come off or someone left an extra one in the differential.

Just wild guessing and probably dead wrong to boot, what about a broken spider gear pinion shaft bolt.

How about the driveshaft slip yoke into the rear of the transmission?


I’d check the u joints first (here’s hoping). Is it an LSD? May have to remove the cover and see what’s up…

I’ll go with Texases…

We have some winners!

Just to finish it off, sometimes the noise sounded and could be felt in the tail shaft (yoke) more than the diff end.

Pulled driveshaft and there it was. U joints toasted on both ends. The ping was coming from one broken cap, the piece floating around and occasionally binding it up.

What threw me initially was the sheet metal like sound at first. Never heard one do that. And then the ping sound which was being transmitted so well by the hollow driveshaft that it felt like something in the rear end.

Like ok4450 and texasas mentioned, I was hoping it was not the ring/pinion but that initial sound still had me wracking my brain trying to envision what could cause it.

Luckily, it was a $40 fix. And one of those weird ones I won’t forget (hopefully).

Thanks for playing along!

Btw- tester it also had the classic symptoms of feeling like being bumped from behind coming to a stop since I bought it. I knew the yoke needed to be greased but not enough of an annoyance to make me do it. Early warning sign ignored…