Originally Posted by sestivers

You guys are wrong. The sensor on my CatEye makes an faint, audible "tick" upon every revolution as the sensor sends its electrical "count" signal to the computer. It is definitely not making physical contact between the sensor and magnet. It cannot be heard when riding, but it can if I pick up the front of the bike and spin the front wheel by hand.

My guess is that this sensor's pulse is stronger than others, so it's more audible. You could probably hear the same thing with a stethoscope on others. I'm sure that a physicist or electrical engineer could explain why there is some sound caused by the magnet interaction with the sensor.