If car-based diesel installations become mainstream I believe we will see significant noise reductions. Partly because liquid cooling makes a lot of difference, but more because these engines will all be geared, and a geared engine can swing a bigger / and/or more-bladed prop at much lower speeds to deliver the same or better thrust as a small prop on a direct-drive engine. This is the most effective way of reducing prop noise which, as others suggest, is the dominant source. However, I believe exhaust design can make a valuable contribution to noise reduction too.