Trading off one car for one drone would result in a massive increase in noise. To be a win, you'd need each drone to replace hundreds or thousands of cars, and right now the equation is the other way around.

Cars are getting quieter all the time, too. Tire noise already dominates for most vehicles at city speeds, and EVs improve the picture even further. Diesel trucks are often pretty noise, but they're mild compared to a drone, and electric trucks will drastically cut down on noise.

Not to mention in most places, highways (the places where cars make the most noise) are far away or acoustically shielded from where there are people.

You’re assuming nothing else would fill the gap left. It’s not a static system.