My neighbour has a W220 S320 CDI and when he starts it up to go to work in the mornings, you don't really hear it. When he comes home from work with the engine all warm, you don't hear it. The S-Class has sound deadening materials around the engine bay and they apparently work. Diesel clatter with an SL400 CDI shouldn't be an issue.
The issue at hand is PERCEPTION. A diesel SL may not sound as appealing today, but in a few years, you'll probably have people begging for it. Diesel is on a rise. Audi, BMW and MB and the French and Italians have excellent diesels. Their popularity is not only gaining because they are efficient, but because they're now actually FUN TO DRIVE. Still, a diesel engine in a car like the SL is as of now a bit "iffy". Give it time, and I am sure it'll be a smash hit when the time is right.