Question about Anesthesia?

Today I got an endoscopy. They had to sedate me, so they put the stuff to do so into my IV and told me "I'm putting the stuff in to your IV right now. You'll feel your arm start to go tingly where the IV needle is, and then you'll hear a whooshing noise then you'll fall asleep." He was right, that was exactly what happened and everything went fine. But I was wondering, what was the whooshing noise? I was going to ask him before I blacked out but it all happened too fast and when I woke up I was pretty disorientated. It was unlike anything that I've ever experienced and I just thought it was a weird feeling. Does anyone know what caused the whooshing noise?