Transmission Problems

[Cole97978945]

I have a 2015 Chevy Silverado 1500 LTZ with a 6 speed transmission. The truck has just rolled over 108,000 miles and I'm starting to have trouble with the transmission. When I'm driving anywhere from 65 mph on up i start to see movement in the rpm guage and i can hear the engine rev up. But when i put the truck into Manual mode where I can shift though the gears I can go all the way up to 5th gear and it works perfect. But if I go into 6th gear the rpms jump rev up and down and it make a deep whistling sound at a Certain rpm. I was just wondering if it was the torque converter or if the overdrive in the transmission is going out?

[1SLOW1500]

Guild flush and new filter.

[Black02Silverado]

10 hours ago, Cole97978945 said:

I have a 2015 Chevy Silverado 1500 LTZ with a 6 speed transmission. The truck has just rolled over 108,000 miles and I'm starting to have trouble with the transmission. When I'm driving anywhere from 65 mph on up i start to see movement in the rpm guage and i can hear the engine rev up. But when i put the truck into Manual mode where I can shift though the gears I can go all the way up to 5th gear and it works perfect. But if I go into 6th gear the rpms jump rev up and down and it make a deep whistling sound at a Certain rpm. I was just wondering if it was the torque converter or if the overdrive in the transmission is going out?

Has the fluid/filter ever been changed?  I would start there as stated above.