Hoping someone out there has seen this one. I have 28K on a 2011 6.7L Lariat. While accelerating normally (every day driving with no trailer or additional weght on truck), when in 3rd gear the RPMs increase as expected but then "decrease" a couple of 100 RPMs momentarily (see it on tach and hear it/feel it) and then increases again prior to shift into 4th. This only happens in 3rd gear. I have never seen this with a tranny before. It feels and sounds like this new tranny is "shifting" within the gear itself if that makes any sense. I had TSP 11-04-21 flash installed several months ago due to some hard shifting at lower gears and seemed to resolved problem. I have yet to take it in to dealer for this one because I wanted to research a bit before turning truck's fate over to service dept. Thanks.
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Sounds like the torque converter locking up (which it starts doing in 3rd).

 

Yes, it's the torque converter locking up. It's normal and working as designed.

 

Thanks guys. Glad to know it is not an issue that needs to be addressed.

 

That is what I was hoping to find. Came searching for that exact same question.