Cloister Bell

The Cloister Bell is an alarm bell located in the Cloister Room inside the TARDIS. The Cloister Bell is to be used in the most serious and utmost emergencies. Its distinctive, sonorous ringing sounds resembles that of a large church bell. It could be answered by anyone inside the TARDIS, and the Doctor told Adric that if he needed him, Adric could sound the Cloister Bell. (DW: Logopolis)

Instances when the Cloister Bell is heard

 * This entry needs confirmation. I will amend as soon as I have positive information; First heard at the end of DW: The Hand of Fear, (this is the reason The Doctor left Sarah Jane Smith in Aberdeen as he was not allowed to take her to Gallifrey), and the begining of DW:The Deadly Assassin.
 * As a harbinger of the Doctor's impending regeneration. (DW: Logopolis)
 * 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: The TV Movie)
 * As the Doctor began to fly the TARDIS erratically (DW: Children in Need Special).
 * When the Master cannibalized the Doctor's TARDIS into a Paradox Machine. (DW: The Sound of Drums)
 * When the TARDIS of the Tenth Doctor and one of his Fifth Doctor crashed together. (DW: Time Crash)
 * After both a marketplace on the planet Shan Shen and the TARDIS exterior were affected by the Bad Wolf entity. The TARDIS may also have been warning the Doctor of the impending theft of the Earth by the Daleks and Davros, the merging of the Doctor and his then companion Donna Noble, or Donna's "death". (DW: Turn Left)

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)