There's no sound you can actually *hear* when it's completely silent. That's the definition of silence: the absence of sound.
However, what people often describe as the sound they hear in complete silence is actually:
* The sound of their own blood rushing through their ears. This is a constant, low-level sound that's usually masked by other noises. In a completely quiet room, you might become more aware of it.
* The brain's own activity. Our brains are constantly processing information, and this activity can sometimes be perceived as a faint humming or ringing sound.
* The sounds of their own breathing and heartbeat. These sounds can be very subtle, but they can become noticeable in a truly silent environment.
So, while there's no "sound" in complete silence, there are often internal sensations and physical noises that our brains interpret as sound.