When Foo Fighters' Dave Grohl stated, "There’s one [guest] who is probably the biggest pop star in the world. He or she sings backup on one of the heaviest songs on the record. And we’re not telling anybody who it is" and added that that person was someone who had been around for a while, some might have considered the legendary Paul McCartney as a possibility. Well, it turns out that McCartney is on the upcoming Foo Fighters disc Concrete and Gold, but is not the mystery pop star that Grohl has been teasing.

Speaking with ET Canada, Grohl confirmed McCartney's appearance, revealing that the legendary Beatles singer-bassist actually sits in behind the drum kit. “Paul McCartney plays drums on one of our songs. He’s a pal. We’ve known him for a long time. He’s great. He’s the most wonderful person in the world. He’s a great guy,” Grohl said.

“He hadn’t even heard of the song. He comes in and Dave picked up an acoustic and showed him real quick. He sat on his special drum set that his tech set up for him. I sat there with a drumstick conducting. He did two takes,” adds drummer Taylor Hawkins.

As for that mystery pop star on the album, the band still hasn't spilled the beans, but Grohl did rule out Adele and Taylor Swift, two popular guesses amongst fans. He revealed that the person in question who guests on the album was in the same studio where the band was recording and asked to be part of the disc. After a day to discuss, Grohl and producer Greg Kurstin found a good backup part on one of the songs and had planned to keep it their "own dirty little secret" until Grohl opened his mouth.

In addition to McCartney, it's been revealed that the Bird and the Bee's Inara George, Dave Koz and the Kills' Alison Mosshart are among those appearing on the disc. Concrete and Gold is due Sept. 15.

Foo Fighters are ramping up their touring as the new album nears. The band just announced a special aftershow taking place Aug. 4 at Chicago's Metro as part of their Lollapalooza weekend festivities.

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