Sorry for the hijack...but it's a year-old thread, and fits my situation exactly.
(2015 GMC Sierra Denali, 6L80, 148,000 miles.)
Bought my truck with 115,000 miles on it, and had the transmission serviced, including dropping the pan for new fluid. No issues were reported. 20,000 miles later I started to have severe low gear hunting, as well as intermittent rumble strip noises. A trip to the tranny shop revealed much shaving carnage, resulting in a complete rebuild, stem to stern, $4,500, with quality individual parts. The shop explained the problem, and indicated they see it often with the 6L80.
All was fine for several months, until it was not. Repeated but intermittent rumble strip noises kept happening. Back to the tranny shop, where they swapped out both the torque converter and the valve body techum assembly. Again, fine for several months, but the rumble strip noises (TC lock-out?) persisted, as well as some noticeable low-gear hunting.
In an out-of-state location from my tranny shop, we collaborated to find a shop that did an HPtune. All was well on the test drive, I was confident in the discussion with the HPtuner guy, and anxious to drive 800 miles back home. But, alas, while no problems occured below 50 mph, the DAMN rumble strip noises were now occurring at 55, 65, and even 75 mph...depending on rpms and terrain pitch...all the way home!
My original shop (out-of state in another direction) intends to get it right, but I am quite frustrated. Until I can bring it back to them (probably for a third torque converter!), they recommended a Motocraft product called Friction Modifier Additive. Although it appears to be helping, I am not confident how long it may last, or if it actually cures the problem.
In the above discussion, the guy asks if tuning off the lock-out on gears 1-4 works so well, why can't you do the same for 5 & 6?
Is the proper fix to buy the beefier torque converter, or replace it with the 6L90?
First post, so thanks in advance for any opinions or advice on my dilemma.
I love the truck, but am seriously considering a trade-in.