2020 GMC AT4 Rear Leaf Spring Slap or Clunk Noise

ploughboy · September 15, 2022

Hi All,

I have a 2020 GMC AT4 Diesel and it just recently started having a clunking sound, most noticeable at low speeds over uneven terrain. I took it to the dealer and they asked if I had them do an undercoating. I told them I had it done locally about a month ago. They were clearly wondering if they had to fix the problem or if it was my fault! They then said that the vehicle's spring and frame should not be undercoated as the coating causes the spring spacers to be ineffective (at first they said the undercoating caused the spacers to deteriorate) causing the springs to slap, making the noise I hear. From what I gather this has been a problem with GMC springs for years. Do they ever plan a fix? I was told to power wash the springs, dry and then grease them like crazy. It seems so odd that such a basic problem wouldn't have forced GM to do a fix/recall?

Any advice would be appreciated. I love the truck but the noise may force me to get rid of it!

Thanks!

kylant · September 15, 2022

that is bogus response from a lazy ass dealer. there is even a GM bulletin out about the noise.

Another JR · September 16, 2022

On 9/15/2022 at 8:05 AM, kylant said:

that is bogus response from a lazy ass dealer. there is even a GM bulletin out about the noise.

Can you tell us the number of that bulletin?

Edited September 16, 2022 by Another JR

Stand By · September 21, 2022

Any chance it could be the rear end locking and unlocking?

leolkfrm · October 4, 2022

this is what the dealer did to my 2022, made no change and i think worse at times, thurs its going back to get all of it cleaned off, the specs are small stones stuck in the wheel bearing grease

Edited October 4, 2022 by leolkfrm

ploughboy · October 6, 2022

Hi All, Thanks for your replies.

When I asked what the fix for the knowing was, the dealer here quoted me $1,600 to replace the rear springs. When it was clear I wasn't going there, they said they would recommend power washing the springs and greasing them like crazy so they don't flap. Looks like that idea would be worse than undercoating oil which doesn't seem to attract road gravel. Initially, they said my undercoating degraded the rubber spacers on the springs. However, the rubber spacers are still there and look to be in perfect shape. They did check the rear end which is where I thought the problem was and said no, it was simply the springs.

After looking around on the net, I found "Spring Things", basically a heavy piece of rubber with a heavy duty zip tie to hold the rubber between the springs for $45. I will give that a try after power washing the springs first and let you know how it goes.

What amazes me about this is that the problem has been ongoing for GM for years and they haven't done anything about it! Instead, they say there is a bulletin to dealers saying don't allow undercoating on the springs. Better to let them just rust? Very strange! The truck is still very comfortable, but makes a lot of noise at low speed now.

You would think that a truck they ask what $80,000 or more for now would be better built? I also have a 2002 extend cab 2500 which I get undercoated annually and never have had any problems with the springs.

Thanks!

kylant · October 6, 2022

On 10/3/2022 at 6:53 PM, leolkfrm said:

this is what the dealer did to my 2022, made no change and i think worse at times, thurs its going back to get all of it cleaned off, the specs are small stones stuck in the wheel bearing grease

this is garbage.

the service department and the tech who did this should be ashamed of themselves

leolkfrm · October 6, 2022

just got back from the dealer, gm, new spring packs ordered, but per gm tech no guarantee that it will fix it or that the issue will not return.

kylant · October 7, 2022

more than likely, it is the bushings making the noise

NaturallyAspirated · October 10, 2022

I'm loving my truck more and more everyday. But... my gasser does this as well. Been planning to take my mechanic friend for a ride before taking it to the dealership, rather than letting the dealership do the diagnosis. Rather let my friend do the investigation and then let the dealership do the fixin!

My truck started having a whine at startup after about 4000 mi. I dunno but kinda seems like a steering pump thing. My friends' gonna be figuring that one out too I guess.

If I learn anything to help you all, will be sharing a post or two.

leolkfrm · November 4, 2022

On 10/7/2022 at 9:44 AM, kylant said:

more than likely, it is the bushings making the noise

might be, as i can shake it sideways and get the noise

leolkfrm · November 13, 2022

so they replaced both spring packs and shackles, no more noise for now

ploughboy · November 18, 2022

I ordered the "Spring Thing " for $45 but haven't installed them yet. In the meantime, I'm living with the noise which only happens at slow speed when just getting going, not when underway at speed. Still amazed that undercoating would result in this problem which the dealer didn't offer to fix as it was my fault for undercoating the truck! Will report on the "Spring Thing" fix after I get her up on the lift. Too busy getting ready for winter here and thinking about another dealership!

Hoginedgewood · November 26, 2022

I'm in americas salt belt, years ago i learned to get undercoated, the dealership would send it out for undercat before you took posesion, this would have been a warranty issue for me, I assume they know what not to spray.

Aaron f · March 1, 2023

I just bought a 2023 gmc 1500 duramax at4 carbon pro. Has about 230miles on it now and I'm starting to get the clunking noise as well. Don't want to deal with the dealers bs and I dang sure don't want to spend the money to fix it myself. Just figured I'd let yall know the problem is still happening the new vics.