My advice? get used to it.. Cuz as you say, NVH inside is great.. Though you could put your funds into better NVH on the inside.. So no matter what insulation you do for the engine bay, you still can't block that grill in the front for air intake and cooling. So sound will still escape from there. Be like moi, I've grown to enjoy the sound.. My SO too, says she knows when I'm around to pick her up.. Cuz she looks up expecting to see a lorry and there's none, just a sexy looking beast on the road..   

I think Mazda's diesel engine is one of the quietest diesel engine around if not the quietest... so if you want it to be really quiet then i think you're looking for the wrong engine tbh... better get hybrid or electric engines.... 😅

Look fr jack liu rm4xx for bonnet sound proofing