In mid September The Foo Fighters will be releasing their ninth studio album Concrete and Gold, which will feature a huge but currently secret popstar that lead singer Dave Grohl won’t confirm until the release.

“We decided we were gonna make it our own dirty little secret and see if anyone can figure it out,” Grohl told Rolling Stone Magazine.

Fans have taken up the challenge, and were quick to start making guesses about who the mystery artist could be. Recently, rumors have swirled around the fandom that the unknown collaborator is either Adele since her producer Greg Kurstin worked on the upcoming LP with the Foo Fighters, or Taylor Swift. But while Dave Grohl thinks Adele is “an amazing person” and T-Swift is “rad,” he confirmed that these two were definitely not on the album.

Grohl has, however, given fans at least a little something to work with, after admitting he’s talked about having the mystery person on the next season of their docu-series Sonic Highways.

He also stated “this person’s been around a long time. And I think I’ve met this person maybe a few times, but I’d never spent time with this person, and we had a blast together. Really fucking fun. And [he or she is] very talented – more so than I ever knew. But, of course, I fucking ran my mouth off; now I have to answer to it every time I pick up the telephone and I’m like, ‘Fuck! I shouldn’t have said anything.”

Our guess here at MXDWN? Justin Timberlake, or even Lady Gaga since the singer has been known to collaborate with artists outside of the pop realm including icons Tony Bennett, and Metallica. Still, no one will really know until the Concrete and Gold release on Sept. 15, so be on the lookout.

The group have already debuted several new songs from the upcoming album including “Dirty Water,” while at a show in Paris and “Sunday Rain” during an encore in Riga, Latvia.

This year on Halloween weekend the Foo Fighters will be performing at the Voodoo Music and Arts Festival in City Park, New Orleans alongside Kendrick Lamar and LCD Soundsystem.