I have a 2008 GMC
Sierra 1500 crew cab z71. A few months ago, I noticed it was making a strange noise while maintaining a steady throttle while driving down the hwy. I even notice it when in cruise mode. It seems to go away when I speed up or slow down. It almost seems as if the truck shift and is lagging but it happens even at higher speeds. It is a low rubble. Any ideas? I had the transmission fluid changed by the dealer at 47000 and has 60000 now started around 57000. Also, I think it started after I shifted the truck from 2wheel to 4 hi and back. Took to dealer and they say 4x would register problem if it didn't shift back properly.
Appreciate the help