I think there needs to be a little more warning with raids if the frequency is indeterminable. In terraria there are a couple of raid events that say "an army is approaching from the west" and says that a couple times before event starts. I propose something similar. Examples:
Eikthyr raid, trampling can be heard
Greydarwves, tree leaves are rustling
Trolls, a tremble in the distance
Swamp, something putrid is coming
Skeletons, eh it's called skeleton surprise maybe leave that as is lol
Drakes, the air is cooling
Bats, screeching in the distance
Fulings, giggling and laughter approaches