Im writing a book and there are many loud screeches in it from these aliens... How would I write the screech?

Just say the aliens screeched. "The fierce screech lasted around 30 seconds, drowning out everyone else's voice." Why make things complicated? You don't want to do SCREEEEECH, because it's unprofessional. Sometimes you can get away with that in kids books, but normally people get annoyed with it.