What type of sound do infrared devices emit?

When sending large files from one computer to another via infrared, some people hear a faint to moderately-loud noise which they describe as a high-pitched, irritating tone with steady rhythm. Not all people are capable of hearing this, yet many claim to feel a rhythmic throbbing in their hearts and heads ociated with the infrared waves. What sort of sound is this (is it infrasound, ultrasound, or normal sound) and how come many people are unable to hear it?