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) were in danger. Its distinctive, sonorous ringing sounds resembles that of a large church bell. It could be answered by anyone inside the TARDIS, and the Fourth Doctor told Adric that if he needed him, Adric could sound the Cloister Bell. (DW: Logopolis)

Instances when the Cloister Bell is 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)
 * Vortizores attack 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)

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)
 * Oddly, the Cloister Bell was not activated (or, for whatever reason, not heard) when the TARDIS was about to explode. (DW: The Pandorica / The Big Bang)