When the beautiful babble of rap music starts pouring from open car windows, that’s how you know it’s summer.

And when the rappers behind that babble are inventing their own language out of squeaks and groans, that’s how you know rap music has changed. Forever? Or just for now? That’s still up for discussion, but here’s what isn’t: Rap’s magic is currently found in the voice, not on the lyric sheet.

Some of the summer’s best groaning is coming from Future, an Atlanta native who spent the past three years climbing his way toward hip-hop’s A-list by vocalizing as if his words were dying inside his mouth. His sophomore effort, “Honest,” has been sliding down Billboard’s albums chart since debuting at No. 2 in April, but it still feels like the season’s agenda-setter — a dozen songs where the 30-year-old’s consonants crumble like stale cookies, allowing his Auto-Tuned vowels to transport all kinds of miscellaneous emotional cargo.

Future is currently touring in support of his sophomore album, "Honest." (Photo:Rankin) Future is currently touring in support of his sophomore album, “Honest.” (Photo:Rankin)

But as magnetic as Future can be, the most radical voice on the radio this summer belongs to Young Thug. He’s a 21-year-old anomaly from Atlanta who seems bent on stretching the genre’s expressive possibilities by utilizing every timbre available to his larynx. “Danny Glover,” a delirious track from his January mixtape “Black Portland,” has slowly oozed its way onto the airwaves as a remix featuring fellow eccentric Nicki Minaj. Next to Young Thug’s staccato mewls, she’s plain Jane.

Admittedly, it isn’t all that hard to stand out from today’s most legible hip-hop hit-makers. Jay Z and Drake are two very different superstars, but their preeminence has plenty to do with their eloquence. And how else to describe the success of 2 Chainz, a rapper whose shouty elocution makes him sound like he’s trying to order a pizza through bad cell reception?

The squishy syllables currently leaking from the mouths of Young Thug and Future feel like a tacit rejection of rapping as we used to know it: verses that depended on enunciated rhymes, reliable rhythms and clever metaphors to tell coherent stories. These guys, along with a raft of peers and imitators, have scrapped those values in favor of immediacy and improvisation — a point that hip-hop has been pushing toward for more than a decade, ever since Southern rap drawled its way to mainstream dominance.

But few have slithered their way into the conversation by rapping like Young Thug. On his ear-goosing 2013 mixtape, “1017 Thug,” he resembled a rowdy tweener going through an exceptionally challenging puberty, rapping about drugs and jewelry in unprecedented yelps. But over the past year, he’s developed a certain precision — and without surrendering his unpredictability. Now he sounds like free jazz.

You can hear it in the warbles of “Danny Glover.” Or in the quasi-verbal slurry of “Stoner,” his other fantastic radio hit. And especially on “Bricks,” a standout track from “Young Thugga Mane La Flare,” a mixtape he released in April with Gucci Mane — another maestro who, compared to Young Thug, now comes across as yesterday’s weirdo.

The song begins with Young Thug acting out an aggravated assault in falsetto hiccups, then veers off into a sequence of quirky street boasts in the style of his hero, Lil Wayne. Lyrically, he’s working at rap’s default setting — sex, drugs, violence — but his actual words are secondary to the fact that they’re built out of whimpers, snarls, yammers and yawps. He sounds two dozen different kinds of alive.

The moods don’t swing as erratically in Future’s new music, but “Honest” still has remarkable emotive breadth. He’s a tough-guy first, but a softie foremost — and he’s never been more bruised than on “Special,” a ballad about being let down by the one you love. Or maybe it’s about the complexities of fame. Or maybe it’s about something else. Future’s words can push a song in one direction while his delivery tugs it in another.

But onstage at the Fillmore in Silver Spring last week, Future skipped “Special” for the hits, barking out pummeling radio singles (“Move That Dope”), eerie mixtape favorites (“Chosen One”) and roughed-up power ballads (“Turn On The Lights”).

It sounded like 60 minutes of highly enthusiastic throat-clearing. But everyone in the house knew how to sing along.