From the description given, the sensor would contain a reed switch. These are extremely simple magnetically activated electrical switches. They can be very small. They are also used in some security setups where a movement that is unwanted causes a magnetic field to either close the switch or open the switch (depending on the type of logic being used). Generally they are silent. I have been able to here some of the larger ones activate and deactivate. It's most likely that you just happen to have one that is making a small noise as it is magnetically closed and opened. The next ten people with the same computer might not hear anything at all.