When The Hollow Sentinel enters the battlefield, Intruder Alarm triggers, untapping all creatures.
Resolve the Whisper ability, returning a creature card from your graveyard to the battlefield.
When the creature enters the battlefield, Intruder Alarm triggers, untapping all creatures.
Activate Whisper by tapping it and sacrificing The Hollow Sentinel and another creature you control, returning Gulping Scraptrap from your graveyard to the battlefield.
When Gulping Scraptrap enters the battlefield, it and Intruder Alarm trigger.
Resolve the Intruder Alarm trigger, untapping all creatures.
Resolve the Gulping Scraptrap trigger, causing you to proliferate.