on 3; From chapter 6

As his hand touched the back door's handle, he heard a last whisper from behind him.

"Hermione Granger."

"What?" Harry said, his hand still on the door.

"Look for a first-year girl named Hermione Granger on the train to Hogwarts."

"Who is she?"

There was no answer, and when Harry turned around, Professor McGonagall was gone.

Seems clear to me that the whisper came from McGonagall - Harry was talking to her, Harry turned his back and heard a whisper from "her" that sounded like her. Harry thinks so to - in chapter 8 we have;

The boy's mouth was hanging open. "Were you told to wait for Harry Potter on the train to Hogwarts, or something like that?"

"No," Hermione said. "Who told you about me? "

"Professor McGonagall and I believe I see why.