Frontman Dave Grohl has revealed that the new Foo Fighters upcoming album Concrete and Gold will feature a number of guests, including "the biggest pop star in the world," but the band is keeping mum on their identity. The album was announced just days ago.

"There's one who is probably the biggest pop star in the world and — I'm not kidding — sings backup on one of the heaviest songs on the record and we're not telling anybody who it is," Grohl teased in a recent interview with BBC One. Many fans believe Adele may be the secret weapon, given her producer Greg Kurstin also produced Concrete and Gold. 

"Without telling anybody, we've been in the studio for the last six months and we made the biggest-sounding Foo Fighters record we've ever made," Grohl continued. "Honestly, I think it's going to take a lot of people by surprise. Every band says it's the best record they've ever made, but wait till you hear it. It's definitely the hugest thing we've ever done."

In another interview, Grohl revealed they chose Kurstin precisely because he had never made a heavy rock record before, and because the Foo Fighters never worked with a pop producer before, saying they liked the idea of combining "our noise and Greg’s big brain and all of his sophisticated arrangements and composition."

Concrete and Gold, the Foo Fighters' ninth studio album, is due out Sept. 15. "Run," the album's first single and video, is already out. The band has also premiered other songs from the album live.

The Foo Fighters will launch a North American tour behind the album, beginning Oct. 7.

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