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 of emergencies, which saw either the TARDIS or its inhabitants 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 of the Cloister Bell ringing
The TARDIS mistakenly believed it was being attacked by the Pus Creatures of Rakos as it materialised in Magnus Greel's bunker. While the Fourth Doctor slept, Romana II apologised for the interruption that it had caused. (PROSE: Under Reykjavik) Later in the Fourth Doctor's life, as a harbinger of the Doctor's impending regeneration at the hands of The Master and as a warning of his actions nearly destroying the universe, the Cloister Bell is heard. (TV: Logopolis) During the Fifth Doctor's very early life, the Cloister Bell rang as the TARDIS was travelling back in time to Event One. (TV: Castrovalva). Later, when an established time corridor, through which the TARDIS was travelling, began collapsing, the Cloister Bell also rang. (AUDIO: The Mutant Phase) Similarly, it sounded when the TARDIS was trapped in a Dalek time corridor. (TV: Resurrection of the Daleks) As a harbinger of the Seventh Doctor's impending encounter with the Timewyrm and when the Doctor started to time ram her, the Cloister Bell sounded. (PROSE: Timewyrm: Genesys) Later in the same adventure, the Cloister Bell rang as the Doctor took the TARDIS into his own mind. (PROSE: Timewyrm: Revelation) Later in the Seventh Doctor's travels, the Cloister Bell indicated the TARDIS' distress after a link to the Eye of Harmony was damaged. (PROSE: Cat's Cradle: Witch Mark) It was also heard when the Seventh Doctor was being called by the Sirens of Time. (AUDIO: The Sirens of Time) The Cloister Bell rang out near the end of the Master's scheme to capture the first seven incarnations of the Doctor, when it was revealed that they were never freed. (GAME: Destiny of the Doctors) Just prior to the destruction of the TARDIS by the Selachians in an alternate timeline created by Elizabeth Klein, the Cloister Bell sounded. (AUDIO: The Architects of History) During the beginning of the Eighth Doctor's life, the Master opened the Eye of Harmony in an attempt to take the Doctor's remaining regenerations. (TV: Doctor Who) The Cloister Bell was also heard through the Eighth Doctor's regenerations, including his travels with Lucie Miller. When Vortisaurs attacked the TARDIS whilst in flight in the Vortex, it was heard. (AUDIO: No More Lies)

During his travels with Samantha Jones, the Cloister Bell rang when the unstable alternative timeline created by the Tractites began collapsing, taking the multiverse with it. (PROSE: Genocide)

It could also be heard by the Eighth Doctor and crew during Ed Hill's attempt at disrupting the Eye of Harmony. (PROSE: Revolution Man)

As the Tenth Doctor began to fly the TARDIS erratically and at ever-increasing speed, the TARDIS showed its distress by ringing the Cloister Bell. (TV: Children in Need Special) Also, when cannibalised the Tenth Doctor's TARDIS, turning it into a paradox machine, the same distress is heard. (TV: The Sound of Drums)

When the Tenth Doctor and Fifth Doctor's TARDISes crashed and merged together, the TARDIS rang the Cloister Bell. (TV: Time Crash) The Tenth Doctor's TARDIS also used it as a warning of the impending "end of the universe". (TV: Turn Left)

During his travels alone and following the destruction of Bowie Base One, the Tenth Doctor heard the Cloister Bell inside the TARDIS as his own demise loomed, this led to the projection of Ood Sigma as a warning of his inevitable fate (TV: The Waters of Mars) and was also heard when the TARDIS chicks appeared in the TARDIS. (COMIC: Terror in the TARDIS)

During the Eleventh Doctor's life, the Cloister Bell sounded as the TARDIS' engines were in danger of phasing out of existence after his regeneration energy had destroyed the console room. (TV: The Eleventh Hour)

In his later life, the Eleventh Doctor heard the Cloister Bell when the Entity was interfering with the TARDIS' systems. (GAME: TARDIS) They also sounded 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, the TARDIS showed its distress by using the Bell. (TV: The Doctor's Wife) It was also heard 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, it was also heard. (GAME: The Eternity Clock) Other moments include the Eleventh Doctor was being trapped in a pocket universe (TV: Hide) and when the Van Baalen Bros.' magnetic hobble-field kidnapped the TARDIS. (TV: Journey to the Centre of the TARDIS)

During the end of the Eleventh Doctor's life, it was heard as the Great Intelligence entered the Doctor's tomb and again when the Great Intelligence entered the Doctor's time stream. (TV: The Name of the Doctor) 

During the beginning of his life, the Twelfth Doctor fainted after regenerating, causing the TARDIS to ring the Cloister Bell. (TV: Deep Breath) Later in his life, it was heard as the Doctor was unconscious in the TARDIS. With the TARDIS under attack from an unknown creature from the end of the universe, Clara recognised the sound straight away. (TV: Listen).  

Behind the scenes

 * The sound of the Cloister Bell was made by hitting a gong in a pool of water.
 * The Bell was not heard on-screen until towards the end of the Fourth Doctor's life and was not heard during other incidents of danger during the following episodes:
 * The looped death of the TARDIS (TV: The Pandorica Opens/The Big Bang) and the activation of the HADS when the TARDIS felt it was under attack. (TV: Cold War)