The Sun reports that McCartney has been secretly working with Cobain's former bandmates, Foo Fighters singer Dave Grohl and bassist Krist Novoselic, in preparation for a Nirvana reunion almost 20 years in the making.
McCartney told The Sun he called up Grohl to "jam with some mates" for a studio session, where they went on to "just make something up."
That "something" is rumored to be the song the guys will play at Wednesday's show.
"I didn't really know who they were," McCartney said. "They are saying how good it is to be back together. I said, Woah? You guys haven't played together for all that time?
"And somebody whispered to me, 'That's Nirvana. You're Kurt.' I couldn't believe it."
Cobain committed suicide in 1994.
The Foo Fighter's Twitter feed has been active as well dropping hints at something special.
So...you guys busy tonight?— Foo Fighters (@foofighters) December 12, 2012
We have something very special for you tonight NYC... twitter.com/foofighters/st…— Foo Fighters (@foofighters) December 12, 2012
Bruce Springsteen is slated to kick off the show at Madison Square Garden. Bon Jovi, Eric Clapton, Billy Joel, Alicia Keys, Chris Martin, the Rolling Stones, Eddie Vedder, Roger Waters, Kanye West and The Who are all expected to be in the lineup.
As many as 2 billion people could tune in to watch the concert live, beginning at 7:30 p.m. ET, on television, at special movie theater events, and streaming online.