Second gen / 2018 Mazda CX-9 2.5T - knocking sound after shutting off engine

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2018 Mazda CX-9 Grand Touring / Deep Crystal Blue / Sand leather
You may need to turn up your volume to hear this.

Anyone hear this on their vehicles? The weather has been really cold here (regularly in the single digits or negatives) but I've never had an engine make knocking noises like this when shutting it off in these temperatures, or at all, before. This was after a 25-30 minute commute and the heat was blowing warm so the engine definitely got up to normal operating temps. If this is normal, fine, but it's weird and I have a hard time believing it should be.

I've always run my CX-9 on premium gas, maybe a few tanks on regular, but that shouldn't matter since the engine takes either. I otherwise don't rev the crap out of the engine, especially not in this cold weather and I don't push the vehicle if the engine hasn't warmed up.

Any theories on this noise? I otherwise plan to call the dealership if the issue persists.
 
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2021 CX-9 Sig
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2021 CX-5 GT
Dude, I don't; know if it is is me, but I don't hear anything. Anyway you could amplify the sound somehow?
 

speed3chris1

2021 Signature CX-9
Couldn't hear much but since you have a 2018 I am going to assume the car is still under warranty. Have the dealer check it out.
 
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2018 Mazda CX-9 Grand Touring / Deep Crystal Blue / Sand leather
This video should be better: https://youtu.be/SkPiJX4_joc (I updated the link in my original post as well)

I used iMovie on my phone to increase the volume by 500% (at least that's what the app said, haha). I don't have anything on my PC for editing videos to increase their volume. You may still need to turn up your volume and/or wear headphones to hear the sound, but it is louder than the original video.

I called the dealership anyway to have them look at the issue just in case. Their initial theory is rapid cooling of the exhaust/catalytic converter, but they'll take a look at it anyway.
 
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2018 CX-9 Signature
I can definitely hear it in the video. It almost sounds like someone fell down the stairs in the distant background. Does this sound occur every single time you shut off? Or just in cold weather?
I can say I never heard that sound when I shut off but then again it doesn't get cold here.
 
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2018 Mazda CX-9 Grand Touring / Deep Crystal Blue / Sand leather
I can definitely hear it in the video. It almost sounds like someone fell down the stairs in the distant background. Does this sound occur every single time you shut off? Or just in cold weather?
I can say I never heard that sound when I shut off but then again it doesn't get cold here.

This has only happened in cold weather so far. The fact that the engine and vehicle generally continues to run smoothly leads me to believe that this likely isn't a major issue, but we'll see what the dealership says after they look at it on Monday. It won't get warm here for a looong while, so ruling out cold temps won't happen for some time. That said, it could be particularly cold temps that lead to this occurring but maybe not temps in the teens or 20s or so and up.

I'll post back with any updates on if this persists (or doesn't) and what the dealership says.
 
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2021 CX-9 Sig
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2021 CX-5 GT
This video should be better: https://youtu.be/SkPiJX4_joc (I updated the link in my original post as well)

I used iMovie on my phone to increase the volume by 500% (at least that's what the app said, haha). I don't have anything on my PC for editing videos to increase their volume. You may still need to turn up your volume and/or wear headphones to hear the sound, but it is louder than the original video.

I called the dealership anyway to have them look at the issue just in case. Their initial theory is rapid cooling of the exhaust/catalytic converter, but they'll take a look at it anyway.

Much better but I am not sure what the heck it is! Almost like a fan coming to a hard stop.
 
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2018 Mazda CX-9 Grand Touring / Deep Crystal Blue / Sand leather
Well, it was a bit warmer (relatively speaking) today, around 10 degrees, and I didn't hear the noise after driving out to run an errand at Target this morning. I tried to listen for it again when I got home and the sound was there, but it was really faint. This leads me to believe that the sound occurs in more extreme cold, but I'll have the dealership look at it anyway to be safe and because it's under warranty.
 
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Mazda CX-9 Signature
My advice to you when you get to the dealership is to request a tech to ride along so you can replicate and point out the noise. Otherwise they will just write it off unable to replicate and waste your time.
 

sm1ke

Work In Progress..
Moderator
Contributor
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Canada
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'18 CX-9 Signature
I know exactly what you're talking about. Don't even need to watch the video. For me, the knocks sound like they're coming from an area close to the firewall, somewhere behind the engine.

I only noticed the sound in the extreme cold, and by extreme, I'm talking -45c and colder. This was concerning to me as well, and I'm planning to take it in, but it went away as it got warmer. Since there are no CELs, I put it on the backburner until I can find the time to get to the dealer. If you get a response before I do, please let us know.


I did a little Googling and came up with a possible answer from a different online forum:

"Also a slight knocking clinging sound is normal upon shutting. As it's the exhaust system contracting after it's expansion during the heat up with the engine heat. It's a normal sound. Other knocking is not normal. Try a higher gasoline octane to limit engine knock & run-on after shut down."

This kind of makes sense when you factor in the colder temps cooling the exhaust system faster than a shut down in warmer temps. I'm still going to contact my dealer about this though.
 
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2018 Mazda CX-9 Grand Touring / Deep Crystal Blue / Sand leather
Update:
The dealer took a look and provided this comment: "THERMAL COEFFICIENT EXPANSION OF ISOTROPIC MATERIALS"

I'm not an engineer or physicist, but in simpler terms: The issue is what they originally thought it was, rapid cooling of hot components after shutting off the engine in particularly cold temps. I'd be interested to know if others living in colder climates are experiencing this.

Otherwise, if this turns into a bigger issue, I at least have it documented that they told me it was okay. That said, with relatively warmer temps today (in the 20s Fahrenheit), I didn't hear the noise at all. Seems to be most noticeable when the temps are around or below 0.
 
Update:
The dealer took a look and provided this comment: "THERMAL COEFFICIENT EXPANSION OF ISOTROPIC MATERIALS"

I'm not an engineer or physicist, but in simpler terms: The issue is what they originally thought it was, rapid cooling of hot components after shutting off the engine in particularly cold temps. I'd be interested to know if others living in colder climates are experiencing this.

Otherwise, if this turns into a bigger issue, I at least have it documented that they told me it was okay. That said, with relatively warmer temps today (in the 20s Fahrenheit), I didn't hear the noise at all. Seems to be most noticeable when the temps are around or below 0.
Hi did you get any issues with the CX9 after ?
My concern is I have exactly the same problem I I cannot locate the sound
 

sm1ke

Work In Progress..
Moderator
Contributor
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Canada
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'18 CX-9 Signature
Oh I forgot to post an update to this thread. I'm not the OP, but I have the same knocking sound. My dealer thought it was an engine mount, so I took it in and they apparently replaced it. It definitely was not the engine mount as the noise was still there. I meant to raise the issue with them again but the weather warmed up and the noise goes away with warmer weather, and then I forgot about it until it started to get colder again. I'm just going to write it off as normal based on @norsairius' post above.
 
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Mazda CX-9 Signature
I wonder if you could better pinpoint the noise if you prop the hood open and have someone turn the car off while you like for it
 

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