Why do Renault car engines sound like they do? -- they've had the same kind of hum since at least the 1970s?

They don't sound the same. There's so many different engines OHV,SOHC,DOHC, inline 4, V6 , 2 valves per cylinder, 4 valves per cylinder etc... Some cars have cheap replacement mufflers that are noisy while others have added a performance exhaust with a raspier sound. Maybe your local muffler man is using the same crappy el cheapo mufflers on the cars in your neck of the woods.

I have noticed that a lot of cars sound the same ie all mitsubishi vehicles have the same sound when they start. I have had three and can pick one starting in car park without even having to lift my head to see what type of car it is. I have an Renault too and have driven others and they too sound the same. I think it might be a manufacturers way of putting their signature on the car..