Sirius screams this high pitched, i-just-found-a-dead-body-in-my-bed kinda scream. Even when Remus sneaks up behind him and hugs him, Sirius will let out an ear piercing shriek because he gets so spooked. Of course, then he pretends that didn't happen and lets Remus hug him and snuggle with him or whatever.
Remus doesn't scream when he gets surprised. He's one of those people who will punch whoever snuck up on them, even if it's their best friend. That's why Sirius makes sure Remus knows when he's there. For a scrawny boy, Remus has a good fist.