Beating Drums

One day, our protagonist notices a faint beating of something that sounds like drums being hit. Large, hollow, deep drums that only he can hear. What could this be?

One day Henry, an early 20 something college student, got ready for class like any other day. He was blessed with the opportunity to be in a dorm room on campus, so getting to his class was as easy as going down a hundred feet to the next building. He got his pre-packed bag, grabbed his water bottle for hydration, and went down to his class like any other day would go. 
Today he decided to skip his usual breakfast however, since Henry was going to meet up with a friend after class at a local cafe. This made him get to class a little earlier than usual, even if just by a few minutes. He sat down, opened his journal, and looked at his E-Watch for the time (Henry's new year resolution was less phone time).

9:15. 5 Minutes before class started; other classmates started walking in and sitting down, with their own respective notebooks or other similar writing devices. This is when he started hearing something off in the distance. He couldn't quite make it out, but there was some sort of occasional off-beat rhythm to it. After the 5 minutes of waiting for class to start, the rhythm stopped just as fast as it started. Henry thought,
"Probably someone playing music in another room, or a construction site in the distance."

And he thought nothing more of it throughout the day. Why would he after all? 
His day went like normal after that occurrence; he went out to eat with his friend, did schoolwork when he got back to his dorm, and after a dinner of leftovers from earlier he decided to go to sleep early. 

The next day rolls in, and this time he decided to get his usual breakfast bagel before class. He grabs the bought bagel, and while walking to class he glances at a clock in the hallway. 9:15, just enough time to make it. 
But ever so slightly louder (yet noticeably so), right on beat that rhythm started again. And no matter how far he walked away from where it started at that time, it sounded as if it was constantly the same distance away. Even if strange, Henry chalked it up to "someone's daily pre-class song", maybe even for a history class or something. And again, just like the day before, it stopped at 9:20.
Even though he stopped walking for a moment when he heard it start, it didn't delay him from coming in on time for class. Even if just barely. 

His day goes by normally. He does schoolwork, goes to bed, wakes up. Everything per usual. This time even managing to get to class a little early with the bagel. This time he's expecting the beat to start, however. He eats takes a bite of the bagel, and just on time it starts; something is wrong this time though. 
The beating, or better yet drumming is twice as loud. Just managing to prevent himself from choking on the bagel from the sudden noise, henry looks around to see where the noise is coming from. At this point, maybe only 4 or 5 (out of 20) other students are sat down, with some just walking in. Clearly no-one is making this drumming noise, especially when its beat by beat the same as the other days. It stops on time, where Henry is very obviously shaken up. 

This goes on for 3 more days, where on the third Henry swears it's as if he's having large drums being beat right next to him. Each day it gets louder, adding to Henry's paranoia since no matter who he asked they couldn't hear what he does. 
Nearly a week later, Henry decided to go to the on-campus urgent care facility. On that day it luckily wasn't crowded, since he decided to get to it around 9:00. His reasoning is that if the nurse (or whoever is taking care of him) can be there when the beating is happening, they can maybe see some sort of physical occurrence inside the ear. After all, there must be some sort of rational reason for this, especially when no one else hears it.

The secretary at the desk tells him to go to the first room on the right, 1A. He walks over and sees a nice and tidy doctor's office. Lots of swabs, replacement nozzles for the ear-viewing devices, wax paper to sit on, and plenty more usual doctor office stuff. So, he sits down on the "patient chair", and lays back onto it as if it were a sofa. 

"I'm gonna get this checked out" he thought to himself, "I just hope whoever comes here does before it ends."
With a small smile of relief, he closes his eyes a little and sighs, since he thinks it's going to be figured out.

In a split second with his eyes still closed, the ambiance in the room changes. From a soft warm hum of the iridescent tube lights found in the doctor's office, to a chittering buzzing from a small weak bulb. The patient chair suddenly lost its give, feeling much harder. And the air was quite cold now. Henry opened his eyes. He was now in a 7 x 7-foot metal box with no openings, and one single dangling light. 
For obvious reasons he was stricken with panic and bolted up. He started hitting the walls of the box, hoping someone can hear him, or even if there's some opening or weak point. This goes on for a few minutes.
Where, a thought suddenly hits him as he's banging on the walls. This rhythm he's been hearing has been the same sounds of him hitting these walls. The same noise, except from the outside. 
But, another thought very quickly crossed his mind nearing 5 minutes of him hitting the wall, 9:19 on his watch. One that shook his being, for it was unavoidable if things are following the same rhythm. As the light from the bulb flickers in and out gradually more and more.

The drumming sounds he heard.
They stop.