K2500 6.5 4X4 Suburban Strange Noise

So I am getting a grinding noise out of my suburban. It happens when accelerating after hitting a bump or dip in the road.

Details- Mostly at lower speeds. I've never heard it on the freeway or anytime cruising.

It happens after things warm up. Almost never when it is cold.

There is not a loss of power and the noise stops quickly if I let off the accelerator.

It is a harsh sound like gears grinding or a metal on metal rub.

I did check the differential fluid and it is full and the fluid looks clean. I believe this vehicle has the posi-traction rear end so I don't know if the previous owner changed the fluid and did not put in the posi-traction modifier???

It will make the noise when going straight, and/or turning left/right. Going over a bump is almost always the cause of the noise.

I honestly can't tell if it is coming from the front or the back of the vehicle. We have had it up and the lift and the driveshafts and u-joints look fine.

Any suggestion?

Can you run it on the lift with all 4 tires in the air?
Happen in both 4 WD and 2WD?
Any chance it's as simple as a rock caught between the rotor and dust shields on the disc brakes?

It happens in 2wd. Will try 4wd and will try it on the lift running.

No rock that I can see caught anywhere.

If you have "the posi-traction rear end" you don't have to worry about any additive. The option was "G80" which is actually a locking differential and no additive is called for.

If the noise happens after hitting a bump and accelerating, I'd look for something, like the exhaust, that can shift and touch the body of the car, then shift back when you let off.

Check to confirm that the fan is not hitting the shroud. I have seen soft or broken motor and transmission mounts cause noise like you describe.
Good luck
John

Perhaps the fan!!! But it does sound like a grinding more than the fan hitting the shroud. Thanks for all the responses so far!

Something is rubbing/hitting the chassis/body

Exhaust is number 1...
As mentioned.....engine mounts ...Both sides and the rear one

Try a stall test...(Tranny in D foot on brake and bring engine speed up to load the drive train....

See if the noise comes in.....

ENGINE OFF
Get a light and inspect the fan shroud for damage (Fan been chewing on things)
Inspect the area where the down pipe from the turbo passes between the frame and the engine near the firewall at the floor area.

Any and all reaches of the exhaust system from the engine all the way to the tail pipe...

Grab the tail pipe and wiggle it vigorously to listen for things hitting (Engine off) system warmed up....

The bump thing makes me think of exhaust hitting .....

Gear train does not start making noise just because you hit a bump.....Power on causes things to shift due to torque.....

Get the beast on a rack with the wheels on the rack (LIKE IT WAS ON THE GROUND) Flashlight in hand and go on a scavenger hunt.....Grab the drive shaft and yank it side to side....

Anything unusual happen just prior to the noise starting.....

I've had two things cause the "grinding" noise when I had my GMT400 trucks. Like said before, the down pipe being one. So I ditched the stock exhaust and went with the larger down to 5" out to stacks. Remember Robyn?;)

The other one that drove me batcrap crazy and was hard to find, was the inspection cover to the trans had three broken bolt tabs on the mounts. It wasn't grinding but rattling and sounded like a grinding noise. Hit a bump and it would make noise. Decelerate and it would stop. Taking off from a stop, it would make noise. Took me forever to find and once I did, it all made sense.