I've been in my world for a while now when every now and then I hear random sounds coming from hostile mobs, which sound like they are right by me but I can't see them, mainly creepers and zombies. I hear the hiss a creeper makes right before it blows up, but nothing blows up and then it starts the hiss again. Also, the normal sound zombies make. I know that they aren't coming from a cave or anywhere where I couldn't see them. If I move away I don't hear them anymore, but it seems like they follow me because I start hearing them again. 

The version I'm playing on is 1.14.4 Java Edition, but it wouldn't let me submit this bug report for 1.14.4, only 1.15.1.