Clunking noise front end


#1

I have an 02 Avalanche 1500 4wd z71. It has 173000 miles. Noticed a clunking noise under driver side when slowly taking off. If I floor it there is no sound at all. This only happens when vehicle is on 2wd mode. If I put it on auto 4wd there is no more clunking. The noise sound like two pcs of bare metal are hitting each other. Any ideas what this could be.


#2

Hard to say. It could be motor mounts, suspension system, drive axels, differentials. Since it seems to come under the driver side, and only in 2wd mode, seems more like it might be a suspension problem on the driver-side front. Have a shop put it up on the rack, I’ll bet there’s something loose, like a worn bushing or something.


#3

Thanks. Forgot to mention that I go over bumps and pot holes on purpose and there is no noise or clunking.


#4

When your car is in 2wd, how is it configured? Is it driven by the rear wheels, using a long driveshaft from the transfer case to a rear pumpkin-like differential??


#5

Most people try to avoid bumps and potholes.


#6

Yes it is driven by the rear wheels exactly the way you described it. Regarding the bumps and small pot holes was just a few ones just to see if the noise was present since I could not find anything loose on the front end. But suspension wise everything seems intact.


#7

OK, I understand you believe the suspension isn’t the cause … and the clunk is never heard in 4wd, but clearly heard in 2wd. And only noticed in 2wd when slowly pulling away from a stop, but not when gunning the engine from a stop, like racing away from a bank heist. Been watching too much tv … lol

hmmm … well, the evidence points to the rear wheel drive components. U joints in the drive shaft, or a little play in the differential. I don’t have an explanation why it is worse when slowly breaking away from a stop, other than maybe there is a lot of other noise that masks it when racing away from a stop quickly. Squealing tires, sirens, police shooting guns … my my, too much tv again … lol

I can add this personal experience, hopefully might provide you some clues. I have an older Ford 4wd truck that exhibits a similar symptom. A bit of a clunk sound when pulling away from a stop. And sometimes when the auto transmission shifts from 1st to second and the engine is cold. I get a little clunk when coming to a stop with the transmission still in drive too.

What’s causing all that on my truck is a little play in the differential. Combined probably with an incorrect ratio of differential fluid to limited slip clutch friction modifier additive, so the differential clutches aren’t slipping as smoothly as they should.

If I had the problem you are describing, I’d do what it takes to make it safe to crawl under the vehicle and grab hold of the driveshaft w/my grubby mitts and see if there’s any play while I twist on it this way and that. If there is some noticeable play, it is likely either in a u-joint or the differential. If you look carefully you can usually see if a u-joint has play. Any noticeable play in a u-joint means the u-joint must be replaced. Some amount of play in the differential is normal. If you decide the problem is play in the differential, you’d need to bring in an expert to evaluate the situation.