So I've recently rescued a 10 month old cat. Nobody really knew her background and only got told that she's aggressive. The foster home I got her from said she's not aggressive at all. Then in the next breath said to ignore her if she hisses at me. She's been with us for nearly a week now. I see progress in her, she will explore at night. She goes and sits on our chair in the living room and I have found her on our bed sleeping when I come in from work. However most of the time when I talk or go near her she will start making a strange sniffing noise followed by hissing and growling. It's very strange to me too because she has came over before and sat on my lap. I stroked her then and for some reason she lashed out hissing and growling at my face. But wouldn't get off my lap. I don't know if it's the case of her needing time to settle more or if it's the case of her actually being an aggressive cat? I don't want an aggressive cat so if she doesn't change her ways after so long then I will have to take her back. I feel terrible about it but I know I am not the person to help her with this behavioural problem (if this is the case).
Give her time. She's in new place that's unfamiliar for her. However, the fact she is hissing suggests she had been hurt by people in the past. Over time she will understand that she can trust you, however that might take some time.