system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: What is that deafening feeling in your ears when you stretch?
gpt
This is actually fascinating process. 

First off, your inner ear is an air chamber filled with sensitive instruments and sealed off from the world by your ear drum. It is, however, connected through your Eustachian tubes to your sinus, allowing it to equalize with changing air pressures. When you have a cold and these tubes get blocked off, diving under water or flying in an airplane can create pressure changes that are truly painful. You need to "pop" your ears to make it go away, or massage the Eustachians by chewing some gum. (They are effected by moving your jaw around)


Now what does this have to do with stretching? When you yawn, your jaw goes way back and your Eustachian tubes open up really wide. (They're not normally wide open, otherwise you'd feel it in your ears every time you swallow.) And then you pinch them off. As you pinch them off, the continued yawning and stretching actions squeeze the tissues around your ear, and your tongue will back against your nasal passage changing the air pressure in your sinus. All of this causes flutters of pressure changes within your inner ear, as you open it, seal it off, squeeze it and release. This momentary pressure in your inner ear pushes against the ear drum and messes with your ability to pick up sound waves during the stretch. 

Now sometimes something really awesome happens. 
Your ear actually has a built-in hearing aid, and it can get a feedback loop and have to reset!
There are tiny hairs(?) in your inner ear that will vibrate in response to sound, amplifying it. Now, just like when someone gets that horrible feedback squeal between a microphone and speakers, sometimes these hairs pick up a random vibration and runaway with it. You'll actually hear a very high-pitched ringing that slowly fades away to silence, with diminished hearing ability for a few minutes until the system has reset.
Stretching sometimes sets off this feedback loop of hearing-aid hairs and can cause you to lose hearing volume for a couple minutes. More commonly you may just need to "pop" your ears after yawning to release any pressure that didn't come back out.  (Do this by pushing your jaw back and swallowing)