Is rennet haram in grated hard cheese?

So inshallah, rennet is permissible, but you just have to bear in mind that there are two reasons, and I'll explain this. The first view says that it's permissible because rennet is taken from a part of the animal which is not considered to be a living part of the animal as such. So even if the animal is not slaughtered in an Islamic way, then it would be permissible to use. And also, based on narrations from the companions of the Prophet Muhammad ﷺ, who ate cheese from the Zoroastrians, and the Zoroastrians would not have slaughtered the animal in a halal manner as such. The second view says that no, it's not permissible because the animal was not slaughtered in a halal way, and also because the narrations they say refers to those of the Christian faith rather than the Zoroastrians. So that's why there's two views. If you're not comfortable, you can always avoid it, but inshallah, based on the first view, it would be permissible.