Locals told The Sun there's a "real buzz" at the home and most are sceptical about claims the party is in honour of Brad's parents' 50th wedding anniversary.
"Brad flew in to oversee preparations but they're all here now," said a source.
"They say it's for Jane and Bill [Pitt], but some think this could be it. Even Angelina's private jeweller is here."
“[Brad's] at his house in Italy, but could fly to Marseille in an hour. There’s a real buzz around the estate.”
The couple, who started dating after they met on the set of the 2005 flick Mr and Mrs Smith, confirmed their engagement in April.
They have six kids together.
According to the Sun, paparazzi have made quick tracks to the couple's $60 million French estate in light of the wedding rumours, but some reporters claim there has been no sign of preparations.