My cat make a weird spitting noise!

I have two cats they were both strays that I found in my apartment hallway. They play together alot and the bigger one gets a little rough. My kitten starts making this spitting noise. I was wondering what causes this noise. What are some of the reasons?

You have to be more specific.
   
What's she doing when the spitting noise happens?
   
Is she puffed up? Arching her back? Angry? Is there a hissing sound also? Are her ears laid back?
   
Scared?
   
Just sitting there?
   
How loud is it? How does the other cat react?
   
Short of an MP3 of the sound, we may have some trouble helping you out.
   
(Come to think of it, try surfing cat sounds & see if there are any of them in MP3 form, or maybe as a cell phone ring, & see if any of them match.
   

Sorry about the not enough information. Let me try again. Ok about 2 months ago I found a stray cat in my apt. hallway. I decided to keep him, I named him Oliver he is a male. He was not fixed at the time. So a couple weeks ago I found another stray cat in my hallway a female who was not spayed, I named her Bella. She was in heat at the time so we kept them seperated in different rooms. Well I have gotten them both fixed now so I put them together. Oliver is twice her size and he is a bit of a bully. He even tries to beat me up on a regular basis. I had him aged at the vet they said he is about 1 1/2 years old. He hates to be picked up and he only lets you pet him occasionally. When we got Bella spayed the said she was a year old. She is a very small cat very lovable likes to be petted and held. The only thing is she is scared of everything she jumps with every noise. Bella loves to try to play with Oliver she tries to get him to chase her and when he does and he catches her he gets extremely rough it's like he tries to eat her. He won't let her go and she starts to hiss and make this really strange noise it's like a spitting noise. Like when a human is about to spit when we release the spit it makes that noise a pffffft kind of noise. I don't really no how else to explain it. She will also do this noise at him when trying to play. I don't know if it is a fight noise or a play noise. I think it always starts out as them playing and then it turns into a fight. I have tried spraying Oliver with a spray bottle when he gets to rough with her but that is only temporary. He always goes back for more sometimes it's Bella that goes back for more and then she ends up getting beat up. Nothing to serious though. I don't know how to get him to be nice to her. Any help would be greatly appreciated if you have any questions just ask. I am in great need of help they do this early in the morning the most so I don't get much sleep. They are fine together when they are tired they will sleep in the same room even next to each other sometimes but at night and the early morning they are constantly engaged in chases and fights.