Hi, this is a fairly low miles G56, does anyone know what is causing this and how involved the fix is? happens in both 5th and 6th gears, less so under load.

 

What year is the truck? My 07 did that exact same thing and it never had a single problem in 210k before I sold it. Chipped, big tires, beat on, work truck and the trans was one of the only things I never had to work on. I always kind of thought it was just a harmonic from the dual mass flywheel driving in too high of a gear. Once I did the southbend clutch and solid flywheel it was too noisy to hear it anymore. I wouldn't spend too much time worrying about it but I know how it's almost impossible to un-hear it once you noticed it.

 

dealer seems to think it should be looked at immediately, haven't been driving the truck so maybe will just bite the bullet and take it in.

it almost seems like the shifter spring that pulls it back into the center gate is a bit on the strong side, when I push the shifter over and relieve the spring pressure the noise goes away.