There are several synonyms to describe the sound of "more hissing". These could include words like "hushing", "whispering", "sizzling", "fizzling", "rustling", "whistling" or even "murmuring". The choice of synonym often depends on the context of the situation in which the sound is heard. For example, "sizzling" might be used to describe the sound of a hot skillet heating up on the stove, while "whispering" might be used to describe the sound of two people murmuring secrets to each other. Whatever the context, there is always a word to aptly describe the sound of "more hissing".