I thought that nightmares were in fact evil monsters that would come and get me through the heating vent. The sound from the heating vent in our old home was a "shuh-shuh,shuh-shuh," and when it came on,my brother would tell me those were the nightmares coming to get me!
A childhood belief about monsters

others believed things like this
- I once thought a vent in my hotel room led down to the center of the earth.— flarn2006
- I believe that there is a monster in the air vent and watches me when I put on my clothes— Blablabla
- eating cheese before bed gives you nightmaresAnon
- I used to believe that a dream catcher really made nightmares go away.
— WOLVERINEfLASH14 - I used to believe that eating chicken skin will give me nightmares.— zihui
- I used to believe that echos were actually the sound going completely around the world and coming back to you.— kyle