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 Perhaps wrong/outdated fluid was used (just a guess)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