Can the drivers and passengers hear the clatter of engine noise while driving down the highway? I am talking about the older diesels, the ones that sound like a Briggs & Stratton engine with the muff...
Even the old diesels are noisier at idle. When running down the road they aren't nearly as loud and are often about the same as a gas equiv. Back in the day I had a Chev. 6.2 (very noisy) and my buddy had a Ford 460. Mine was actually more quiet much of the time (except at idle).