Rattle noise after trans service - only when cold-ish and under light throttle.....hmmm

[B-train]

I have a 2017 yukon 6.2L, 8 speed. I had it in to get a trans flush, LOF, diff service, t-case service, and alignment. I normally do most of my maintenance but figured I'd throw the dealership a bone for helping me on some warranty needs.

So, after getting the truck home and it sat overnight my wife said it made a weird noise in the morning, but that it went away. I test drove it when it was cooled off and got a weird gravely sounding rattle when taking off. It was more repeatable under light throttle and usually only made the noise in 1st and 2nd gear. Once you get moving above a mph I never recorded (duh), it goes away. Once the truck is warm, it's quiet as a mouse again.

I crawled under and banged around on the exhaust. I could hear the cats make a little noise, but I'm not sure they would rattle like that(?) I know the old cats of the 80-90 era that would sound like a box of marbles when they failed. The flex is good, the y-pipe and flanges appear good. I did check the stupid flapper valve, but it's never made noise that I know of.

Am I hearing a torque converter noise? It was silent as a mouse before a trans service. I know in the GM manual it says doing a brake torque is an acceptable method to heat up the trans to check the fluid (stupid in my book, too easy to cook it) Could the tech have F'd up something there?

Could it be:
1. A cat that rattles til warm?
2. Some sort of torque converter or flywheel issue?
3. Exhaust flapper valve?

I plan on trying to clip the valve open when it cools off, but in reading other posts, that may not keep it from rattling (?)

I'm usually quite good at pinpointing noises, but this one has me stumped.

Insight needed and appreciated. Especially from someone who may know more about the 8 speed than I or has lived through this experience.

Thanks!

 

[NickTransmissions]

To the best of your recollection, about where is the sound coming from? You had a lot of different subsystems touched at once which potentially complicates things….Did they only flush the transmission or was the pan dropped and electronic parts replaced?

Unfortunately, I have yet to work on an 8L or 10L transmission but what’s being described does resemble the symptom pattern for failing TC, per this GEARS Magazine article. Perhaps wrong/outdated fluid was used (just a guess)

Its odd that you would only notice something like this after a service (hence why i do all my own stuff).

 

[B-train]

It was supposed to be a flush. My good friend at the GM dealer wasn't able to do it, so it got passed to a Jr tech. He did state that he's seen guys not check the fluid level after a flush and the customers came back with issues.

I tend to lean towards trans now that I've read other posts. It was quiet and smooth before dropping it off for service.

So, an update from the daily driver (my wife). Apparently it would only do it when cold for a few take offs or back outs. Now recently it will do it more frequently on long drives when warmed up.

I'm going to get in touch with the dealership tomorrow and see what can be done. We've put on about 1300 miles so far and the problem is apparently getting worse. I fear a trans failure is in my future unfortunately

 

[NickTransmissions]

The fill and check level procedure for these 8 and 10 speeds can be tricky and somewhat time consuming, especially if the tech isn’t familiar or experienced on them; also maybe Techs are cutting corners to push stuff out faster (obviously not the right way to do things).

 

[B-train]

That doesn't surprise me. I wish it just had a damn dipstick so I could do it myself and be able to put eyes on it regularly.

Thanks for the reply and advice

 

[B-train]

To the best of your recollection, about where is the sound coming from? You had a lot of different subsystems touched at once which potentially complicates things….Did they only flush the transmission or was the pan dropped and electronic parts replaced?

Unfortunately, I have yet to work on an 8L or 10L transmission but what’s being described does resemble the symptom pattern for failing TC, per this GEARS Magazine article. Perhaps wrong/outdated fluid was used (just a guess)

Its odd that you would only notice something like this after a service (hence why i do all my own stuff).

the sounds seems to come from rught under the console or passenger area. I might have my wife drive me around the yard tonight and I'll hand out the window like a dog

 

[B-train]

Updated:

Took it back to the dealership and they found it 1 quart low. They pulled the pan and inspected I guess and said there was no damage. Here's to hoping all is well for the future. They wouldn't do anything in writing but said if something comes up in the future they will work with me. Normally I wouldn't take that for shit at a dealership, but I do put stake in my friends word. Hopefully I don't need to test it....

As if now the truck is running and shifting properly. I will update if something happens related to this.

 

[OR VietVet]

So after the 1 quart low, is "running and shifting properly" but is the noise gone? C'mon man! You had never complained about the shifting, unless I missed it.

 

[iboughtatahoe23]

@B-train you just explained the same issue I’m having. Did you figure out what it was? For me it’s at 25mph when lightly applying the throttle. Same gravely type grinding noise.

Does yours sound like this at all?

 

[B-train]

@B-train you just explained the same issue I’m having. Did you figure out what it was? For me it’s at 25mph when lightly applying the throttle. Same gravely type grinding noise.

Does yours sound like this at all?

Yes, it was similar to that. Usually from stop and go, or turning and accelerating. It ended up being a quart low on fluid after they did the service......Idiots

 

[iboughtatahoe23]

Yes, it was similar to that. Usually from stop and go, or turning and accelerating. It ended up being a quart low on fluid after they did the service......Idiots

man that’s strange. I had my fluid changed in march and have had some intermittent shifting issues and then that grind noise. Maybe time to call the dealership and make sure they filled it.

I will also check my fluid thanks.

 

[iboughtatahoe23]

man that’s strange. I had my fluid changed in march and have had some intermittent shifting issues and then that grind noise. Maybe time to call the dealership and make sure they filled it.

I will also check my fluid thanks.

And i before I had it changed by dealer I told them I’m having shifting issues. They said my fluid was low and dirty after the salesman told me it was good on fluids. So two different stories there.

I have a hard time believing either.

 

[NickTransmissions]

Yes, it was similar to that. Usually from stop and go, or turning and accelerating. It ended up being a quart low on fluid after they did the service......Idiots

Not to cause panic but I wonder if your pump was partially cavitating which was the noise you were hearing. Once the fluid level was restored to full, it stopped as there was no more air displacing the fluid inside the pump. Typically being only a quart low wouldn't cause this but it's also possible you were a lot more than just one (1) quart shy of full or those units are super sensitive to fluid levels. In your trans, the pump itself is actually set back a bit from the very front of the case, not directly in front like pretty much all GM transmissions made prior so the location of the sound makes sense. Also pumps can make a variety of different sounding noises but the common themes are 'knocking', 'chattering', 'ticking', 'rattling', etc.

Here's the pump relative to the front of the transmission (looks a little like a power steering pump), which is towards the right hand side of the image.

 

[B-train]

Not to cause panic but I wonder if your pump was partially cavitating which was the noise you were hearing. Once the fluid level was restored to full, it stopped as there was no more air displacing the fluid inside the pump. Typically being only a quart low wouldn't cause this but it's also possible you were a lot more than just one (1) quart shy of full or those units are super sensitive to fluid levels. In your trans, the pump itself is actually set back a bit from the very front of the case, not directly in front like pretty much all GM transmissions made prior so the location of the sound makes sense. Also pumps can make a variety of different sounding noises but the common themes are 'knocking', 'chattering', 'ticking', 'rattling', etc.

Here's the pump relative to the front of the transmission (looks a little like a power steering pump), which is towards the right hand side of the image.
View attachment 404714

I would agree with you, but it wasn't constant and would happen when cold and when the fluid sloshed around. It was definitely pump cavitation. This is why I will do my own stuff from now on......not some flat rate bafoon hoping for no come-backs.

2017, 8 speed is what I had the issue on. I'm assuming the same as OP but not certain. It's interesting how sensitive these new transmissions are. I miss the 90's and early 2000's.

Thank you for the pic and explanation BTW!

 

[NickTransmissions]

I would agree with you, but it wasn't constant and would happen when cold and when the fluid sloshed around. It was definitely pump cavitation. This is why I will do my own stuff from now on......not some flat rate bafoon hoping for no come-backs.

2017, 8 speed is what I had the issue on. I'm assuming the same as OP but not certain. It's interesting how sensitive these new transmissions are. I miss the 90's and early 2000's.

Thank you for the pic and explanation BTW!

You're welcome, B-Train and agree - the legacy three and four speeds were a lot less complex from a command and control stand point and flat-raters couldn't care less (ive rebuilt a lot of flat rate jobs from customers trying to save a buck).

I would also assume it's the same for the OP'er as well

 

[iboughtatahoe23]

Also checked my fluid. And it’s good…

I would agree with you, but it wasn't constant and would happen when cold and when the fluid sloshed around. It was definitely pump cavitation. This is why I will do my own stuff from now on......not some flat rate bafoon hoping for no come-backs.

2017, 8 speed is what I had the issue on. I'm assuming the same as OP but not certain. It's interesting how sensitive these new transmissions are. I miss the 90's and early 2000's.

Thank you for the pic and explanation BTW!

So it was just low fluid for you?

 

[B-train]

Yes. The 8 speed has a stand tube, no dipstick (stupid and cheap in my book). When they did the flush, the tech didn't check the level after it was warmed up and it ended up being low. Gotta love flat rate