Dear Folks,
Gears, especially "straight cut" or "spur" gears, tend to make noise. This is true of gears in a Harley tranny, gears in your lawn tractor, gears in the rear axle in your pick-em-up truck, gears in a piece of industrial equipment, etc. The amount of noise that gets transmitted to the outside world will vary depending on the accuracy of the machining of the gear teeth, the actual fit of the gear teeth to each other, and the design of the rest of the component containing the gears. The noise you're hearing in 1st and 5th in a typical Cruise Drive trans is the same noise you would have heard in 1st, 2nd, 3rd, 4th, and 5th in the previous transmission. The fact that the Cruise Drive uses much quieter helical gears for 2nd, 3rd,4th, and 6th is what makes the normal straight cut gear noise in 5th more evident. Everything is relative.
If the noise is that bothersome, throw some wood shavings / saw dust in your trans lube to dampen the noise. That always helped in the old days when trying to sell an old car with a noisy rear end.

Jerry