A perfectly running 6.5 sounds different depending on things like when it is cold and warm.
Mostly, it is clacky at idle. Even more so when cold.
When it is in its sweet-spot of 1,800 - 2,200 K RPM, the clack is mostly non-existant (unless cold or something else wrong calling for advance timing) where it has a typical big block v8 rumble.
Exhaust note has a characteristic whump-whunp due to GM's routing of driver's side stream colliding with the #8, then #6 on its way to the turbo.