The pulse you weren't aware of a second ago
You become aware of your heartbeat. Just slightly faster than you'd like. So you check it. Is it still going? It is. Now you can feel it more. You watch it more closely, and the closer you watch, the louder it gets, until a normal pulse has become the only thing in the room.
Nothing has actually gone wrong with the heart. Something has gone up with the attention. And the two have started feeding each other.
The watching turns the volume up
Under stress, a body signal, a pulse, a breath, a tightness in the chest, gets louder the more you watch it. Attention isn't a neutral observer here. It's an amplifier. You point it at a sensation to check whether the sensation is dangerous, and the pointing makes the sensation bigger, which reads as more dangerous, which pulls more attention. Round it goes.
More body awareness isn't always better. If the reading on your internal instrument is set to alarmed, then turning up the awareness just turns up the alarm. The signal was probably fine. The dial it's being read on was the problem.
A real caveat, kept in its place
One thing worth saying plainly: new or severe symptoms still deserve a proper look. This isn't a reason to ignore your body or skip the doctor. Get the genuinely new, the genuinely severe, properly checked.
This pattern is about the other thing, the familiar sensation that gets enormous under the spotlight of stressed attention. Once a real check has been done and the answer is that the body's fine, the loud signal that returns is the alarm running, not the body failing.
What helps, what backfires
What helps is to orient outward first. Look around the room. Name a few things you can see. Get some graded, gentle movement going, a walk, a stretch, something that puts attention on the world instead of the interior. Outward and moving, before any turn inward.
What backfires is exactly the instinct this pattern produces: intense breath-holds, long body scans, dropping your full focus straight into the loudest sensation. That's pointing the amplifier at the alarm. It reliably makes the sound worse.
What to watch
Run the small experiment. Notice the alarm rating, orient outward and move gently for a few minutes, then check the rating again. If it drops, you've learned something useful about your own wiring: the volume responded to where your attention went, not to anything in the body.
Orient to the room and move gently before you turn inward. The signal you're hearing so clearly is mostly the listening, set a little too loud.