Cloister Bell

The Cloister Bell was an alarm bell located in the Cloister Room inside TARDISes. The Cloister Bell was to be used in the most serious and utmost emergencies, emergencies so grave that even the TARDIS itself (and its inhabitants) was 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 can be manually activated. (DW: Logopolis)

Instances when the Cloister Bell was heard

 * As a harbinger of the Doctor's impending regeneration. (DW: Logopolis, possibly also in DW : The Waters of Mars)
 * When the TARDIS was traveling back in time to Event One. (DW: Castrovalva)
 * When the TARDIS was trapped in the Dalek Time corridor. (DW: Resurrection of the Daleks)
 * When the Master opened the Eye of Harmony in an attempt to take the Eighth Doctor's remaining regenerations, signifying that the TARDIS itself was dying. (DW: Doctor Who)
 * When vortizores attacked the TARDIS whilst in flight in the Vortex. (BBCR/BFA: No More Lies)
 * As the Tenth Doctor began to fly the TARDIS erratically. (DW: Children in Need Special)
 * When the Master cannibalized the Tenth Doctor's TARDIS into a paradox machine. (DW: The Sound of Drums)
 * When the TARDIS of the Tenth Doctor and the TARDIS of the Fifth Doctor crashed together. (DW: Time Crash)
 * When warning of the impending "end of the universe", as the Doctor said. (DW: Turn Left)
 * Following the destruction of Bowie Base One and his vision of Ood Sigma, the Doctor heard the Cloister Bell inside the TARDIS. (DW: The Waters of Mars)
 * Just prior to the destruction of the TARDIS by the Selachians in an alternate timeline created by Elizabeth Klein. (BFA: The Architects of History)
 * When the TARDIS's engines were in danger of phasing out of existence. (DW: The Eleventh Hour)
 * When The Entity was interfering with the TARDIS' systems. (VG: TARDIS)
 * When the Siren teleported the TARDIS aboard her home ship. (DW: The Curse of the Black Spot)
 * When the TARDIS was taken over by House. (DW: The Doctor's Wife)
 * When the Eleventh Doctor opened the door to his 'greatest fear'. (DW: The God Complex)

Other information

 * The Cloister Bell was not activated during an incident in which the Fast Return Switch malfunctioned, threatening the TARDIS and its crew. It's possible the bell was deactivated by the malfunction, or had not yet been installed. (DW: The Edge of Destruction) Nor was it sounding when the TARDIS' doors open just before it materialises, causing it to run out of control, thus making the TARDIS reduced in size. Instead an emergency klaxon is heard. (DW: Planet of Giants)
 * Indeed, the bell is never heard once until towards the end of the Fourth Doctor's life. It's never been indicated if the bell is a piece of original equipment, so it's possible the Fourth Doctor installed it.
 * Oddly, the Cloister Bell was not activated (or, for whatever reason, not heard) when the TARDIS was about to explode. (DW: The Pandorica Opens / The Big Bang)

Behind the scenes

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