Age. And a turbo. New 6.2 trucks were fairly quiet. Today, 6.2 trucks rarely have the original intake plumbing intact. The 6.5TD has a turbo, so it needed less attention in this department. A turbo is a noise reducer, in both directions. Many states don't require an additional muffler (as a means of noise control) if it has a turbo. The N/A 6.5 is every bit as noisy as the N/A 6.2, all else being equal.