Fourth gear is by far the noisy gear in a tko600. Especially if you are lugging it a little. 5th is not straight cut from my fuzzy recolection. I've never noticed much noise in any other gear.
I think it's safe to say that a problem needs to be found with your kugel rear end. Unless they are that weak, it has to be a set up or a manufacturing problem. Of course, it is chromed.
What is the hp rating?