Explosion

Today when I was driving on the highway, I heard a weird flapping noise. If I passed 80 km/h, it got very loud. I wasn't sure if the noise is always there and I just didn't notice before. Or it was something that broke recently. I varied my speed and it seems that if I kept it under 80 km/h, it was fine. But as I drove any faster, the noise got louder and louder. It sound like some air pressure noise, but I didn't open any windows. I start thinking my car may explode in the highway. I can't figure out what happen. I got a bit scared (but I didn't think of stopping my car because I was late already). I started to think that may be it was my timing belt that broke. And hence the flapping noise. I don't know how far I could drive before it broke down. But none of my car's sensors showed anything wrong. I don't know what could happen. Then, when I did my shoulder check, I saw the back window is half opened! THAT IS WHY THE NOISE! But....but I don't remember opening it at all. Then I remember I drove some coworkers home last night and one of them might have opened the window. No wonder when I drove another friend last night, she told me window still opened. I thought she meant the one on the passenger side.