Apple's El Capitan update comes with a force touch technology. This technology allows you to interact with your device by exerting different levels of finger pressure on the touchscreen or trackpad. If you'd like to turn off the clicking sound that accompanies this update, here's a quick tutorial on how to do so.
N.B. This tip only applies to Macs equipped with Force Touch trackpads.
El Capitan's Silent Clicking Option
Click on the Apple menu, and go to your System Preferences. Click on the Trackpad option, followed by Point & Click.
Tick the Silent clicking checkbox to mute the click sounds. You can also clear the Force Click and haptic feedback checkbox to disable the haptics (vibration).