Backyard Fowl Keepers

Sounds like she's broody! The behavior you're describing,sitting on eggs, staying in the nest for long periods, and making noises,is classic broody behavior. Silkies are well-known for being broody. If the cold isn't too intense yet, she's probably okay. Keep an eye on her and if it gets much colder, the heat plate should help. If she's comfortable staying in the nest, let her be for now.

It's more likely that she's broody than cold. Broody hens tend to isolate themselves and sometimes refuse to roost, even in cooler temperatures. It's a good idea to use the heat plate once it gets colder, but I wouldn't worry too much unless she's showing signs of being unwell. If she's comfortable in the nest, let her stay there. She might eventually break her broodiness and join the others.

It's possible that she's broody, especially if she's hissing and sitting on eggs. Broody hens often act like this, and Silkies are known for it. If she's only staying in the nesting box at night and doesn't seem distressed, it's probably not the cold. I'd continue using the brood plate when the weather gets colder, but for now, I'd let her do her thing. Broody hens don't always like to be moved.