Single dull knock sound when accelerating during driving.

[Matheuszh]

Hello everyone,

Im driving Kia Stinger 3.3 2018 AWD with 100k kilometers counted and I have a strange "problem" to which I couldn't find answer on any forum.

So basically, during driving, when car is warmed up (95-100c oil temp.), I'm hearing a strange, dull thud noise coming from bottom of the car, more like back area but I'm not sure, definitely not the engine.

It only happens, when I let go off gas pedal during driving, leave it to roll for some time and press it again. Right after that dull knock sound it starts to accelerate and it DOES NOT happen when changing gears, automatically or via paddles.

Also it happened when I let go of gas at low speed (20kmph), happened only once.

I've asked about that during oil change at KIA, they looked at it thinking its some drive shaft problem but everything seemed fine and told me It is probably normal and to return if it gets louder.
I'm not sure if it actually is because I've had this car for about 8k kilometers (car has about 100k) and noticed it some time after purchase so I don't know if that was a thing from the beginning.

Has anyone experienced something like that?

 

[Buzzrush]

This may be to simple, but try tightening down your spare tire. Mine loosened up and made noise when accelerating hard or braking hard. If that’s not it, I got nuthin.

 

[Matheuszh]

This may be to simple, but try tightening down your spare tire. Mine loosened up and made noise when accelerating hard or braking hard. If that’s not it, I got nuthin.

Definitely not spare tire. Also it happens right before it starts to accelerate.

 

[Buzzrush]

Damn…well hopefully other members here can help you figure it out. Lots of sharp people on this forum. Good luck!

 

[G70LEbyAdoom]

Gotta get it on video

 

[Matheuszh]

Gotta get it on video

I'll try to upload it tomorrow.

Does anyone have any ideas for now?

 

[nhcowboy28]

DB in trunk.

Rookie mistake.

 

[SRV]

I'll try to upload it tomorrow.

Does anyone have any ideas for now?

It could be the rear differential.

I had a car in the past which did the same upon acceleration and decelleration - there was slack in the diff which was taken-up with a clunk upon acceleration, and released it with a clunk when decellerating.

Replaced the diff and all was good.

 

[Rodtheviking]

I would say center support bearing or one of the rubber driveline joints.

 

[Matheuszh]

I have uploaded the video on youtube, every knock you hear, I'm pressing gas pedal and releasing it (knock only upon pressing).

It is definitely loudest in trunk area, I have removed everything from it so for sure its not any dead body inside.

It could also be coming from center of the car but the sound appears to be loudest in the trunk.

Also, my friend suggested it could be exhaust but Im not sure about that.

Any other ideas? I'm going to visit KIA next week and tell them to check some of your suggestions specifically.

 

[oddball]

The Kia techs may not be willing to spend time chasing this as it's just an NVH complaint at this point. And Kia's stance on NVH is "well, the car runs, doesn't it?"

Something under the car is loose or worn out. Folks have already highlighted all the primary contenders: driveshaft center support, diff support, exhaust hangers. Could also be the trans mount or engine mounts.

If you really want to go after it then you'll need to crawl under there and start shaking stuff to see what can move ""too much"", or pay an independent shop to go after it.

 

[Matheuszh]

The Kia techs may not be willing to spend time chasing this as it's just an NVH (Noise, Vibration, and Harshness) complaint at this point. And Kia's stance on NVH (Noise, Vibration, and Harshness) is "well, the car runs, doesn't it?"

Something under the car is loose or worn out. Folks have already highlighted all the primary contenders: driveshaft center support, diff support, exhaust hangers. Could also be the trans mount or engine mounts.

If you really want to go after it then you'll need to crawl under there and start shaking stuff to see what can move ""too much"", or pay an independent shop to go after it.

I'll give them last shot about that, if they won't find anything, I'm gonna go to someone else.

 

[Matheuszh]

Update

Turns out it is rear diff, previous owner wasnt family friendly driver so it started to struggle.

Needs to be replaced in the future.

Rip wallet

 

[Rodtheviking]

Ugh

 

[SRV]

Dang it...

 

[General_Vodka]

Update

Turns out it is rear diff, previous owner wasnt family friendly driver so it started to struggle.

Needs to be replaced in the future.

Rip wallet

Any knocking on deceleration? Seems like a strange issue. What did they say caused it?

 

[Matheuszh]

Any knocking on deceleration? Seems like a strange issue. What did they say caused it?

Yes, it started to sometimes knock on deceleration, they had another stinger to compare it with so they lifted both up. They asked me to come and shown me differences, on mine theres a weird metalic sound when you rotate drive shaft near back diff part, coming exactly from the inside and also there is no resistance when you want to rotate back wheels with hand, unlike the "working" one which it was being compared to.

Guy said it really is strange issue and very rarely something like that happens in stingers so i guess its just combination of bad luck and crazy previous owner. It has the most stress when you're making aggressive turns (drifting, racetracks etc). He made sure to check everything else from things that you said (i didn't tell him) and is 90% sure its rear diff. From what i saw im pretty sure too.

 

[oddball]

So the rear turns like an open diff? That means with the rear wheels in the air and the driveshaft locked, if you turn one wheel forward, the other wheel would turn backward.

Wait, which trim level do you have? In 2018, the LSD only came on the GT2 RWD trim. All other cars had an open diff.

A clutch-type LSD, which has been around for decades, can wear down the clutch plates and turn into an open diff. Kia used a Torsen style which is all gears. So for that to start acting like an open diff would require major damage. Like the rear would be unlikely able to turn at all.

Anyway, it's certainly possible there's some damage in the rear. It's hard to do a visual inspection because of how the diff is mounted, but the fluid can be drained easily to look for chunks.

 

[Matheuszh]

I have 2018 GT2 AWD, the one they compared it to was exactly the same model with same specs (it came as mine was being checked, luckily). When guy spinned my wheel, it was moving freely. The other one (second car) required much more force. I didn't pay attention to second wheel when he was doing that. Also when we spun driveshaft right by diff, there was some weird, subtle, hard to describe sound inside which could match the one when driving when applied force is stronger.

I'm not that experienced in mechanics so I can't make any speculations about what's wrong with that, also they did not drain the oil as you mentioned so I can't say for sure. They've checked everything in back area and couldn't find anything else.

Also, they were checking it for about 3-4 hours to be as sure as they can that it is not something else. Luckily they charged me absurdly low for that. Kudos for them.

EDIT
My bad, they did drain the oil but didn't say anything except that it was not changed since the purchase. Forgot about that.

 

[oddball]

Yeah, the rear diff is probably toast.