So what actually makes the tssssss noise on a turboed car? the waste gate or blow off valve?

i always thought it was the bov, but ive been told recently its the wastegate because my friend has a 2011 wrx and revved it and we were all like man you need a bov on that thing and he was like NO WASTEGATE because it didnt make a loud tssssss when he let off the gas after he revved. i still think its the bov that makes the noise but i need some other thoughts here to help me sort this out. im not a dumb a** kid i know what im talking about ive just had different people telling me different things