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Discussion Starter #1
Good day. Recently bought a used 2017 Duramax with only 30K. Very clean truck . . . was lucky!!! I think!?
It has a 4" Rough Country lift on it, and have already schedule a shop to replace the stabilizer links. Also has a serious "Chevy Lean" to the driver side (wondering if it's okay to shim that side in the front???).
The BIG problem is that, having taken the truck with my RV trailer up into the mountains this last weekend, I noticed an unusual sound when the tranny shifted down. Was going uphill at about 50mph. After several miles of this uphill drive and numerous downshifts, all of a sudden I thought I heard what sounded like a dull and quiet horn when the tranny shifted down. I was able to replicated this sound to confirm, but only one more time before the highway leveled out. I have been unable to replicate this "sound" again, as all my travels since have been on a level highway, or downhill. It only seemed to do this when ascending a steeper grade pulling a load. Any ideas? Is this sound normal?
 

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2018 Colorado ZR2, Duramax, BDS 4" Lift, 35" Nitto Trail Grappler, AEV Snorkel, S&B Cold Air
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I have a 2018 zr2 with the Duramax and just over 80k miles. Have heard the same noise (not towing), but accelerating uphill on three different occasions when it downshifted. I have not been able to pinpoint yet. I did just perform a transmission fluid change yesterday. I'll be interested to see if it goes away.

I would not call it normal, but this is the first truck I've owned that has such a rigorous transmission maintenance schedule. It might be worth looking into changing the fluid - can't hurt IMO.
 

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That kind of sounds like clutch chatter to me. It could be from the TCC slowing back down after the shift or from the shifting clutch itself. It is also sometimes called clutch squawk. What was teh duration of this noise?
 

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Discussion Starter #4
I have a 2018 zr2 with the Duramax and just over 80k miles. Have heard the same noise (not towing), but accelerating uphill on three different occasions when it downshifted. I have not been able to pinpoint yet. I did just perform a transmission fluid change yesterday. I'll be interested to see if it goes away.

I would not call it normal, but this is the first truck I've owned that has such a rigorous transmission maintenance schedule. It might be worth looking into changing the fluid - can't hurt IMO.
I will try changing the tranny oil soon. Let me know if that is working for you. Thanks for the tip!
 

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Discussion Starter #5
That kind of sounds like clutch chatter to me. It could be from the TCC slowing back down after the shift or from the shifting clutch itself. It is also sometimes called clutch squawk. What was teh duration of this noise?
It lasts for just a split second . . . maybe 1/2 second?
 

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Whelp, that does sound like it could be clutch chatter during a high energy lockup. Fluid flush might be a good idea to refresh the additive.
 

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Discussion Starter #7
Whelp, that does sound like it could be clutch chatter during a high energy lockup. Fluid flush might be a good idea to refresh the additive.
Gracious thanks!!! I will give that a shot. I kind of doubt that anyone has changed the tranny fluid at all! I don't think that the dealership would have done it, even though it a "Certified" pre-owned, etc, etc.. By any way, is there any special tranny oil made for longevity or higher performance than what is spec'd out???
 

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Discussion Starter #8
Dang, forgot to ask . . . are there any synthetic motor oils recommended for the Duramax? I am so used to using Mobile 1 in all of my vehicles. Owning a diesel, I just want to make sure I take care of her the best way that I can.
 

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Ac delco dextron VI transmission fluid is the specd fluid for the diesel transmission and is now full synthetic i would stick with that. Also have them do a fluid exchange with a new filter. Not a flush.
 

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I doubt you need a new filter since you're not having any performance issues and a flush with the exchange machine will provide a much more thorough fluid swap than a single pan drop. Total cost is probably about the same, either paying more for labor with the pan swap or paying for more fluid if using the flush machines.

I know the flush machines get a bad rap but I would attribute that more to people getting fluid flushes on cars with 150k that have never seen a fluid swap and then having tranny issues. It's more likely in most cases that the fluid's fresh detergent an dispersant help break loose particulates that subsequently causes issues as opposed to being related to the use of a flush machine specifically.
 

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Discussion Starter #11
Thanks for everyone's input and advise. However, I have done a bit more digging only to find that there are quite a few people out there complaining about this identical noise when downshifting while cruising at 40-50mph. Could this sound have ANYTHING to do with the Centrifugal Pendulum Absorber??? Might have to repost this as a new "topic" to see if I can catch someone's attention.
 

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That damper is meant to absorb engine pulses that are transferred through the converter clutch and prevent them from excessively pulsing the input shaft of the transmission. Excitation of the transmission input shaft by vibration becomes NVH for passengers. Under low engine speed, high torque situations in steady state operation that damper will absorb the torque spikes that are created by individual cylinder firing. That damper certainly has a natural frequency which could be agitated under the right conditions but I would bet it's been tuned well outside the engine's operating frequency. That being said, it's possible that during a shift, poor shift quality could create the right conditions for excitation of the damper. This wouldn't mean that there is anything wrong with the damper though, only that the shift quality is poor and the damper might be susceptible to excitation by it.
 

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Is this post about Katlin Jenner running for governor of Ca?
 
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Discussion Starter #14
That damper is meant to absorb engine pulses that are transferred through the converter clutch and prevent them from excessively pulsing the input shaft of the transmission. Excitation of the transmission input shaft by vibration becomes NVH for passengers. Under low engine speed, high torque situations in steady state operation that damper will absorb the torque spikes that are created by individual cylinder firing. That damper certainly has a natural frequency which could be agitated under the right conditions but I would bet it's been tuned well outside the engine's operating frequency. That being said, it's possible that during a shift, poor shift quality could create the right conditions for excitation of the damper. This wouldn't mean that there is anything wrong with the damper though, only that the shift quality is poor and the damper might be susceptible to excitation by it.
Thank you for taking the time to respond so expansively. So, may I ask, in my situation (and those of many others), could this be the reason for the dull and brief sound heard when the tranny downshifts under load?? Or, should I be worried about the sound that I am hearing? There does not seem to be any degridation to performance.
 

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I would guess your damper is operating as designed, and there is probably nothing wrong with it. I can't say that the damper isn't in any way responsible for the sound as it's a partially contributing factor, lots of moving parts in these systems and their interplay is important. The springs in the damper certainly allow it to contribute to something like this, honestly converter shudder is highly impacted by the damper because the natural frequency of the damper is what basically defines what frequency the shudder will be at IF the fluid allows the converter clutch to stick/slip. Part of the difference is that converters target a constant slip speed, so if you're on the highway, your converter might try to slip at 15RPM and maintain that for 100 miles if you're driving flat, cruise, etc. That's why shudder is like driving on a rumble strip -- It just keeps sticking/slipping until you get the converter to change what it's doing by changing the throttle input.

The damper may be a contributor in the same way to the noise in the shifting clutch. It allows the shifting clutch to chatter during the engagement because the springs in it give it a natural frequency that can be achieved by that downstream clutch in the system. The key here is that can only happen if the low speed friction in the clutch is poor causing the aforementioned stick/slip. What makes it not like shudder is that in the case of the shifting clutch, it only slips for a fraction of a second, thus the short term noise. Better low speed friction control would probably help that clutch not make that noise but in your case it was under more demanding conditions.

If it were pervasive and lots of people were experiencing it under daily driving conditions, I'd be more concerned. A simple flush might help restore the friction modifiers that would prevent that noise in your case. That noise certainly isn't good for the clutch but there's also no guarantee that a flush with fresh DEXRON VI will fix it either.
 

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Discussion Starter #16
Just as an after thought regarding this brief sound that I heard, could it have anything to do with the smog control system??? Anyone in the emissions department weigh in?
 
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