pylon:
Quite a bad sign. I cannot hear my (working) sensor and that's fine because the frequency is 40kHz, more than double of what a human ear is able to detect.
Even though they produce a 40kHz ping, you can still hear them click. Some are louder than others, but I can hear every one of my sensors if I get close enough (some are quite loud). We're not hearing the actual 40kHz ping, there's some type of vibration that's produced that's well within the human audible range. The clicking isn't even that high of a frequency either, it's probably close to 1kHz.
Tim