Cloister Bell

The Cloister Bell was an alarm bell located in the Cloister Room inside TARDISes. The Cloister Bell was used in the most serious emergencies, emergencies so grave that even the TARDIS itself and its inhabitants were in danger. Its distinctive, sonorous, ringing sound resembled that of a large church bell. It could be answered by anyone inside the TARDIS. The Fourth Doctor told Adric that if he needed him, Adric could sound the Cloister Bell, indicating it could be manually activated. (TV: Logopolis)

Instances when the Cloister Bell was heard

 * When the TARDIS mistakenly believed it was being attacked by the Pus Creatures of Rakos when it landed in Magnus Greel's bunker. (PROSE: Under Reykjavik)
 * As a harbinger of the Doctor's impending regeneration/the Master's impending actions nearly destroying the universe. (TV: Logopolis)
 * When the TARDIS was travelling back in time to Event One. (TV: Castrovalva)
 * When the time corridor the TARDIS was in began collapsing. (AUDIO: The Mutant Phase)
 * When the TARDIS was trapped in the Dalek time corridor. (TV: Resurrection of the Daleks)
 * As a harbinger of the Doctor's impending encounter with the Timewyrm, and when the Doctor started to time ram her. (PROSE: Timewyrm: Genesys)
 * When the Doctor took the TARDIS into his own mind. (PROSE: Timewyrm: Revelation)
 * When the TARDIS' link to the Eye of Harmony was damaged. (PROSE: Cat's Cradle: Witch Mark)
 * When the Seventh Doctor was being called by the Sirens of Time. (AUDIO: The Sirens of Time)
 * Near the end of the Master's scheme to capture the first seven incarnations of the Doctor, when it was revealed they were never freed. (GAME: Destiny of the Doctors)
 * When the Master opened the Eye of Harmony in an attempt to take the Eighth Doctor's remaining regenerations. (TV: Doctor Who)
 * When Vortisaurs attacked the TARDIS whilst in flight in the Vortex. (AUDIO: No More Lies)
 * When the unstable alternative timeline created by the Tractites began collapsing, taking the multiverse with it. (PROSE: Genocide)
 * When Ed Hill disrupted the Eye of Harmony. (PROSE: Revolution Man)
 * As the Tenth Doctor began to fly the TARDIS erratically and at ever-increasing speed. (TV: Children in Need Special)
 * When cannibalised the Tenth Doctor's TARDIS into a paradox machine. (TV: The Sound of Drums)
 * When the TARDIS of the Tenth Doctor and the TARDIS of the Fifth Doctor crashed together. (TV: Time Crash)
 * When warning of the impending "end of the universe", as the Tenth Doctor said. (TV: Turn Left)
 * Following the destruction of Bowie Base One and his vision of Ood Sigma, the Tenth Doctor heard the Cloister Bell inside the TARDIS as his own demise loomed. (TV: The Waters of Mars)
 * Just prior to the destruction of the TARDIS by the Selachians in an alternate timeline created by Elizabeth Klein. (AUDIO: The Architects of History)
 * When the TARDIS' engines were in danger of phasing out of existence. (TV: The Eleventh Hour)
 * When the Entity was interfering with the TARDIS' systems. (GAME: TARDIS)
 * When the Siren teleported the TARDIS aboard her home ship. (TV: The Curse of the Black Spot)
 * When the TARDIS was taken over by House. (TV: The Doctor's Wife)
 * When the Eleventh Doctor opened the door to his "greatest fear". (TV: The God Complex)
 * When the TARDIS got caught up by the time storm created by an Eternity Clock. (GAME: The Eternity Clock)
 * When the Eleventh Doctor was trapped in a pocket universe. (TV: Hide)
 * When TARDIS chicks appeared in the TARDIS. (COMIC: Terror in the TARDIS)
 * When the Van Baalen Bros. magnetic hobble-field grabbed the TARDIS. (TV: Journey to the Centre of the TARDIS)
 * When the Eleventh Doctor and the Great Intelligence entered the Doctor's tomb and again when the Great Intelligence entered the Doctor's timestream. (TV: The Name of the Doctor)
 * When the Twelfth Doctor fainted after regenerating (TV: Deep Breath)

Instances when the Cloister Bell was not heard
The bell was never heard until towards the end of the Fourth Doctor's life. Particularly, the Cloister Bell was not activated:
 * during an incident in which the fast return switch malfunctioned, threatening the TARDIS and its crew; (TV: The Edge of Destruction)
 * when the TARDIS' doors opened just before it materialised, causing it to run out of control, making the TARDIS reduced in size. Instead an emergency klaxon was heard; (TV: Planet of Giants)
 * when Turlough took away the space-time element and the TARDIS, threatened with break-up, locked onto the nearest safe place in space-time (in this case, Terminus); (TV: Terminus)
 * when the TARDIS was about to explode; (TV: The Pandorica Opens / The Big Bang)
 * when HADS kicked in and it relocated to the South Pole because it felt it was under attack. (TV: Cold War)

Behind the scenes

 * The sound of the Cloister Bell was made by hitting a gong in a pool of water.