So basically this is what happens when I'm not asleep at midnight.
The banging starts about fifteen minutes after Enjolras stormed out of Grantaire's house. They had been arguing about something stupid, something like "hey Enjolras, your hair's getting a bit long" that turned into something like "I can do what I want, Grantaire, and you have no say" that turned into the exact words "I don't need you". Grantaire felt bitter about it for a couple of minutes before realising how stupid an argument it really was and how it was really all just about Enjolras' stupid hair, and really Enjolras could do what he wanted with it. So he resolved to call him tomorrow and make it up to him.
The banging persists over the next five minutes, and Grantaire begins to wonder who it is. He lives in the suburbs of Paris, and the people who live round there are dangerous. It's probably Montparnasse, trying to break in and take something in return for what Grantaire did a few months back, convincing Éponine that she could live without that creep, that he wouldn't go and murder Gav or Zelma because everyone would know it was him and why murder someone if you know you're going to get caught? So Grantaire sits back and lets the banging continue, because Monty will get too tired to fight soon and wonder off.
When the banging stops, Grantaire frowns. Not that he didn't want it to stop, but he was expecting some kind of kick to the door and a loud curse from Parnasse. But then –
"Gran-antaire!" the yell is strangled, like someone in extreme pain. It's Enjolras. Grantaire leaps up and runs to the door.
Enjolras is outside. He's lying along the driveway in his own blood, an arm that once clutched his side now lying limply across his stomach, next to the stab wound in his side. Grantaire is too late.