Trackpad Locked MacBook Pro
What you are experiencing is the haptic feedback from the Force Touch Trackpad available in your MacBook Pro. Starting 2015, the MacBook (Retina) and MacBook Pro Trackpads include a technology called Force Touch which can be configured to provide haptic feedback. Haptic feedback is experienced only when the MacBook Pro is powered on.

To learn more, refer to the Apple Support article, How to use the Force Touch trackpad.