The 'Best of Me' hitmakers feature both Dave Grohl and Pat Smear - who were both part of the legendary grunge group - and will be performing at the event on the same line-up as former bandmate Krist Novoselic's band Giants In The Trees.
A couple of cryptic tweets have got fans talking, initially after the band responded to an announcement regarding Joan Jett's 'Bad Reputation' documentary airing during the festival in California.
They wrote: ''CAN'T WAIT!!!! What else could we have up our sleeves?? Stay tuned.... #CalJam18''
The 'I Love Rock & Roll' singer joined Nirvana - whose frontman Kurt Cobain tragically took his own life in 1994 aged 27 - as a backing singer when they were inducted into the Rock and Roll Hall of Fame four years ago.
Deer Tick singer John McCauley also performed that night, and he will be part of the bill over the weekend with his own group Deer Tick.
Adding fuel to the fire, just a few hours after the initial tweet, Foo Fighters shared a clip from that very performance.
They simply captioned it: ''This is a Jam @caljamfest#CalJam18 (sic)''
The potential surprise comes after bassist Krist hit out at fans claiming it wasn't a full reunion when he joined Dave and Pat on stage earlier this year to perform The Vaselines' 'Molly's Lips'.
After tweets suggesting it's not the same without Kurt , he sarcastically responded: ''We tried to get a hold of him for this gig -- as many times as we tried to contact Kurt, we couldn't get through. The phone just kept ringing & ringing.
''Kurt does not have email. In fact I have never emailed him in my life and I have been online since 1993.''
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