Some people can make a rumbling sound in their ears by simply tensing a muscle
Take a big yawn. Like, really big.
Did you hear a low, rumbling sound? What just happened was the tensor tympani, a muscle inside your ear, flexed.
You can also achieve a similar effect by pressing your hand against your ear.
But not everyone needs to yawn or stare at Britney in order to hear that rumbling sound. There are some among us who can flex their tensor tympani without making everyone else around them yawn, as science twitter user Massimo pointed out:
When we hear sounds, we're actually hearing vibrations. In order to hear those vibrations, your eardrum vibrates, and this is then transferred to three bones in your ear (your malleus, incus and stapes), then to your inner ear.
Writing for ScienceAlert, Michelle Starr explains how the tensor tympani works to protect our hearing:
The malleus is the closest to the eardrum; it transmits the membrane's vibrations to the incus. And the tensor tympani is connected to the malleus. When it contracts, it pulls the malleus away from the eardrum, which tenses the eardrum membrane (or tympanic membrane, hence the muscle's name), limiting its ability to vibrate and thus dampening the vibrations transmitted through to the inner ear.
The tensor tympani also filters out low frequency sounds, which allows us to hear high frequency sounds better. It also can protect our ears from damage caused by loud noises. The tympanic reflex muffles the vibrations on the tympanic membranes from loud noises. The reaction time for this is 40 milliseconds, so it would protect from things like loud thunder, rather than fireworks or a gunshot.
At the end of her article, Starr also noted that there are people who can equalize the pressure in their ear on command. Some of us are able to "click" our eustachian tubes in our ears. As someone who is able to do this, I can affirm that it is very annoying. And what's worse, sometimes my eustachian tube just stays open.
It just does it on its own sometimes, but can be easily induced through exercise. It sounds almost like being under water, except a bit more muffled. I went to an ENT doctor about this a few years ago when it first started, and after various exams/tests, I was told that my hearing is just fine and it's nothing to worry about. He said there's a tiny layer of fat that helps seal off the tube, and that the fat isn't quite doing its job. Just another reason why bodies are terrible.