There is no need to worry.
You have just been too aware about you normal body functioning.

Really? I thought maybe that was abnormal. So it happens to everyone?

Yeah for the most part normal however the echoing noise might be due to allergies or asthma. Make an appointment with your DR if it is that concerning but imo it sounds totally normal. Oh and if you can hear it more pronounced when your mouth is open chances are its not coming from your lungs, but from your throat and upper respiratory tract. When you breathe through your nose, breath sounds don't vibrate as much and you won't hear much or feel much difference unless you have a stuffed nose, which can cause a slight rattling sound. But when air passes through an open mouth a lot of things can interfere and make a normal breath sound strange. Like say you have a bit of saliva back there, or a bit of mucous, the air passes over that creating a crackle or possibly snoring sound. I used to get so freaked out when my first son was teething because all that drool back there made him sound like an old man dying from pneumonia. All you would hear was this wierd gurgle sound.

I've been experiencing the exact same symptoms for about two and a half years. I went to see my doctor about it but he told me it was nothing to worry about. If you went to get it checked out, I'd love to hear what your doctor told you. Also, did your symptoms ever just go away on their own. You're right about it being a little scary.

You guys, I promise that this is absolutely normal. It happens to a lot of people. Make some of your friends do the same motion you do, and you will discover that many, if not all, experience the exact same thing. No worries, no stress-- don't take life so seriously. You'll never get out alive.