Strange Cat Noises

My cat likes to sit near me when I'm on a computer, preferably next to the keyboard or on my shoulder. In my home office (spare bedroom) she has to jump up and walk across the printer. Which caused one to break! and so I had to buy a new and better one for less money than the broken laser printer - I got a multi-function printer/fax/copier/scanner from HP. Any way, there's a bed in the room too since it's one of our guest rooms. She likes to sit on the edge of the head of the bed and look out the window which goes almost to the floor. The front bushes cover most of the window but I have raised the blinds about one foot and there is a small area where she can see the world from the window. Tonight she was sitting there and started making a weird growling hissing sort of noise. I looked out of the window and a twin black cat with collar was looking in at her. I guess she wasn't pleased. The visitor cat went on his/her way when I went over to look at it. Cat make strange noises!