2015 Silerado, Trans noise when accelerating in 4th gear.

12345lonestar · November 27, 2020

I got a new truck (2015 Silverado Rally II, Automatic 6 speed, 6I80 trans) with 80K miles.

When I first got it, it would have the typical shifting issues mainly around 4th gear while on the highway. It would up shift, down shift erratically and would rev all over the place, and clunk. First time I got it on the highway for an extended trip, it went into limp mode after erratic shifting and clunking in 4th. Reset after turning the truck off and back on. Then about 2 weeks later the truck then began making a screeching,humming noise when in 4th gear and accelerating. It starts off quiet and progressively gets louder as you accelerate up to 2000 RPMs and stops completely if you pull off the gas. It is worse if driving into the wind or uphill. I lose a lot of power in 4th gear. Almost to the point i cant accelerate but RPMs go up. If I push the gas in 4th, it will clunk badly and usually jump into limp mode.

I am 100% certain this is related to the transmission. When in manual at 3, it runs great 1st through 3rd gears. No noise, plenty of power and fairly smooth shifting. The second i move into 4th, it will screech when i use gas, But if I manage to get it into 5th gear, it then runs fine again. i can gas it and get it up to 80-90 mph no problem with no noise.

So it seems to be isolated to the 4th gear.

I checked the fluid, and its pretty dark and smells old.

I am hoping the clutches and plates are ok. But by the sound it seems the 4th gear is not getting enough pressure, and that's why its making the noise and is not getting power. Also the erratic shifting. I'm hoping it may be something as simple of a blocked valve due to dirty fluid or a malfunctioning solenoid.

I have a month left on the 3 month warranty from the dealer, so ill take it in Monday, but id like to have a good idea what may be wrong so I can make sure they look into the proper issue.

So hopefully this is enough information for someone to give me some insight in what might be going on. Thank you.