Foo Fighters’ forthcoming album Concrete and Gold is loaded with special guests, both of the expected (The Kills’ Alison Mosshart) and unexpected (Boyz II Men’s Shawn Stockman) variety. Sirvana collaborator Paul McCartney fits more into the former category — though, he plays drums on the record. And while McCartney is obviously a huge name, the rock legend isn’t “the biggest pop star in the world” Grohl first teased during a June interview with BBC Radio 1’s Annie Mac. Turns out that person is none other than Justin Timberlake.

A new Rolling Stone cover story details the collaboration and how it came about:

“Then there was the day Justin Timberlake dropped by. He listened to what the Foos were doing and liked what he heard, and pretty soon he and Grohl were hanging out regularly. ‘We’d drink whiskey in the parking lot,’ Grohl says. ‘He was really, really cool. Then the night before his last day, he says, ‘Can I sing on your record? I don’t want to push it, but – I just want to be able to tell my friends.’ So the band had him add some ‘la la la’s’ to one track. ‘He nailed it,’ Grohl says. ‘I’m telling you – the guy’s going somewhere.’”

The Rolling Stone feature includes more behind-the-scenes details on the album’s creation as well as Grohl’s thoughts on the passing of Chris Cornell. (Soundgarden was actually booked to play Foo Fighters’ forthcoming CAL JAM festival.) There’s also a funny anecdote about Grohl going to Amoeba Records to buy his daughter vinyl records only to be stymied by a Lana Del Rey in-store performance. Read it in full here.

Concrete and Gold is out September 15th.