Why do cars these days make that ear-busting noise when only one window is rolled down?

It's not new ... it may be that they don't remember it, because their cars didn't go as fast "back in the day".

The air rushing like that occurs much more in newer cars than in older partly because cars used to be driven slower, yes, but also in part because car designers used o be less worried about fuel efficiency. Most older cars are less aerodynamic, meaning less air rushes past the window, which combined with the lower speeds the were driven at, means very few of them made the same noise with only one window down.

I love cars and know them mainly imports but dude I have no clue what you're talking about. turbo? the spools? that is the best sound.

To prevent it from being as noisy, if you open the window that is diagonal to it it prevents it from being as noisy. So if driver window is open, then open back passenger side window

Well, I certainly remember what a roar they made. The wind blew your hair in all directions and the driver's left elbow got a sunburn hanging out the window and the passenger got a wind burned face if the trip was long enough. And you couldn't do anything else because air conditioning hadn't been invented yet. What has changed is that cars used to kill 50,000 people every year, even when the population was half what it is now. Single car wrecks were common, meaning the car left the road and hit a fixed object. Movie star Tom Mix died drunk when his car left the road and a suitcase came off the rear shelf and broke his neck. Drinking while driving was not a crime in those days.