In what was the worst-kept secret on 12-12-12, Paul McCartney did indeed take the stage with the surviving members of Nirvana for the Sandy charity concert. However, fans of the Beatles star or of Dave Grohl, Pat Smear and Krist Novoselic may not recognize the song they played together.

The track is called "Cut Me Some Slack," and was apparently created during a collaboration for Grohl's forthcoming documentary "Sound City." It's definitely a mix of penchant McCartney melody, a heavy rhythm section( just like the Foos frontman likes it) and a heavy dose of that grunge music the kids are always talking about.

Immediately following the rather impressive performance from the quartet, the "Sound City" Twitter account and website released a short clip of audio from the recording, made in California studio after which the film is named. You can hear it below.

"Sound City - Real to Reel" is a compilation described as "music from and inspired by the film," with release date TBA. As for the film -- Grohl's directorial debut -- it gets a premiere at the 2013 Sundance Film Festival (Jan. 17-27) and release on DVD on Feb. 1.

Other artists to feature in the film include Tom Petty, Trent Reznor, Rick Rubin and the ilk, so it should be pretty interesting to see what other combinations Grohl came up with on-screen and on-tape. For now we have... Nirveatle? Paulvana? McGrohl? Kurt Cobain was a big Beatles fan, so we'd like to think he'd approve.

Grohl and Novoselic have combined only a scant few times in the years following Cobain's death. Last year, they celebrated the 20th anniversary of Nirvana's "Nevermind" last year. In April this year, "Nevermind" producer Butch Vig had mentioned he had combined with Grohl and Novoselic and a "special guest" for an unnamed project... so now we know who's likely produced "Cut Me Some Slack" along with just who the "special guest" was.