It would be libelous to insist I know without presenting evidence. What I suspect is, if you were to look to the core English continent of the squad you would not be far away from the leakers. We are not seeing lots of stuff about Rangnick in the Portuguese or Spanish press, for example. Just think of the journalists who are writing this stuff, who they are close to and its not too hard to fit together.
Here's a concrete example: Edu Aguirre, the Spanish journalist, was at Ronaldo's birthday party with Bruno and Dalot lately. You see him in the photos. He's been close to Ronaldo since Madrid. Have you seen Aguirre write anything about Rangnick? If/when he does you will know its source.
Its often suggested that its one or two players who are not getting games. People have, for example, been throwing Henderson and Lingard's names around. However, say for argument's sake that those two are involved. I would suspect they are not alone. Its very easy to blame players on the fringes. However, the reality is that some fan favourites have to be considered too.
As if the likes of Eric Bailly are very happy at United and wouldn't leak a thing to the press.