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Part of the problem is you were not very good at describing what was happening. People were assuming other scenarios.

If it's happening before you switch out of park I suspect it's not transmission or drivetrain related, but no guesses.
If it’s happening 2 seconds after starting it but not all the time, same issue I have, not sure if it only happens to gasoline powered colorados….but I have never owned a vehicle that does this, I asked a few other guys with colorados and they say they never really pay attention to notice something like that….maybe we are over sensitive to a simple issue like this….or maybe the tranny is under filled…or maybe someone forgot a one way valve or plug in some trannys…..we will probably never know…
 

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Discussion Starter · #22 ·
Part of the problem is you were not very good at describing what was happening. People were assuming other scenarios.

If it's happening before you switch out of park I suspect it's not transmission or drivetrain related, but no guesses.
There isn't much else to describe. As I said - "upon start up..." which indicates starting the vehicle. I didn't mention anything about shifting. Thanks again all for the thoughts and commentary.
 

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Part of the problem is you were not very good at describing what was happening. People were assuming other scenarios.

If it's happening before you switch out of park I suspect it's not transmission or drivetrain related, but no guesses.
he described it correctly, it took 7 comments before MrCamo read it correctly then you chimed in again with invalid info. wow
 
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There isn't much else to describe. As I said - "upon start up..." which indicates starting the vehicle. I didn't mention anything about shifting. Thanks again all for the thoughts and commentary.
But "startup" could also mean starting to move--which was my interpretation, and something that does occur and is normal. Others took it to mean shift into second, which is a bit harder to reconcile with "startup," and it also normal.

I just wonder if it could be something totally unrelated to the transmission, such as maybe the electric power steering or some other electric component engaging. The direction a sound is coming from can be very misleading.
 

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Discussion Starter · #26 ·
But "startup" could also mean starting to move--which was my interpretation, and something that does occur and is normal. Others took it to mean shift into second, which is a bit harder to reconcile with "startup," and it also normal.

I just wonder if it could be something totally unrelated to the transmission, such as maybe the electric power steering or some other electric component engaging. The direction a sound is coming from can be very misleading.
Sure it could be relating to something other than the transmission. All I know is it comes from the vicinity of it when it happens. I'm inclined to think it could be a motor mount that is loose (or something...) that because of the torque of everything underneath, causes either the drive train, transmission, etc to hit the underside of the vehicle.

On the other hand - as Hunter 62 said, it could be the torque converter.

I'm just trying to find possible troubleshooting start points because the "mechanics" dont seem to be able to do it.

Thank you
 

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The following condition can most definitely be perceived as a clunk or slight bump.
My 2018 Colorado does this very frequently especially when turning the key to on when the truck has been remote started, but also from a normal start. This bulletin applies to pretty much all GM's from 2015-2021 with the 8 speed transmission.
***Current bulletin
#08-07-30-026G: Information on Slight Vehicle Movement in Park at Start Up After Cold Soak - (Oct 19, 2021)
Subject: Information on Slight Vehicle Movement in Park at Start Up After Cold Soak
Models: 2022 and Prior GM Passenger Cars and Light Duty Trucks
Equipped with Automatic Transmission
Attention: This bulletin also applies to any of the above vehicles that may be Export from North America.
This bulletin has been revised to add the 2021-2022 Model Years. Please discard Corporate Bulletin Number 08-07-30-026F.
Some vehicles may rock or move slightly forward or rearward while in Park at start up after cold soak. This condition may be accompanied by a clunk noise. This is a slight movement that is more often seen visually, rather than felt, when viewed from the outside and using the auto-start feature, if equipped.
This slight movement is due to the residual transmission fluid in the clutch packs that creates a partial apply on start up. This movement is stopped by the parking pawl and/or transmission fluid exhausting completely from the clutch packs.
This condition is normal and no repairs should be attempted.
-The prior bulletin gave a little bit more explanation as to what is/may be happening
***Bulletin #18-NA-285: Information on Slight Movement in Park at Ignition On and Vehicle Start Up and/or During a Remote Vehicle Start Event - (Jan 22, 2021)
"This condition may be caused by the application of the 1-2-7-8 Reverse Clutch and the 1-2-3-4-5 Reverse Clutch in addition to residual transmission fluid in the other clutch packs creating a partial apply on start up.
-The movement may be more apparent at Ignition On, during a remote vehicle start. As the 1-2-7-8 Reverse Clutch and the 1-2-3-4-5 Reverse Clutch are applied at Ignition On, the engine and portions of the transmission are at full idle speed causing the clutches to apply at a quicker rate than during a normal start. This movement is stopped by the park pawl and/or the transmission fluid exhausting completely from the clutch packs."
"This condition is normal and no repairs should be attempted"
I wouldn't worry about it, its just a quirk of the transmission
 

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My truck has started doing this in the past week (give or take). It seems to be weather temp related, as we recently had some cold weather and I'd not noticed it before. It's kind of unsettling, but reading the service bulletin above makes sense.
 

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This slight movement is due to the residual transmission fluid in the clutch packs that creates a partial apply on start up. This movement is stopped by the parking pawl and/or transmission fluid exhausting completely from the clutch packs.
I wonder if you could diagnose it as this by removing the parking brake and having as little pressure on the brake pedal as possible when starting the vehicle. On a flat space, obviously.
 

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Mine has done it most of the 5 + years I have owned it (maybe 4 + years) when the transmission is cold.
If the transmission is "warm" it does not do it... Only takes a few minutes to get fluid all around the transmission when it gets cold to not have the issue. Also find it most often when the truck is raked rearward.
When I am level or raked forward it does not seem to be an issue, And of course not in warm weather.
 

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My 2018 Canyon Diesel just started doing this at 80,000 km. A clunk right after engine start that sounds like drive train or transmission. I recently towed a 5500 lb trailer about 1500km (first time towing). Will try the starting in neutral instead of park to see if it has any effect. but that bulletin sounds like it might what is going on. I would say it's 1 out of 5 starts, and haven't kept track if it is related to cold or warm start.
 

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My 2018 Canyon Diesel just started doing this at 80,000 km. A clunk right after engine start that sounds like drive train or transmission. I recently towed a 5500 lb trailer about 1500km (first time towing). Will try the starting in neutral instead of park to see if it has any effect. but that bulletin sounds like it might what is going on. I would say it's 1 out of 5 starts, and haven't kept track if it is related to cold or warm start.
Sounds like it. . I really don’t notice anymore. Either I have just got use to it or maybe its gone away🤷‍♂️
Just a quirk of this Transmission I guess, (one of a few)
 
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