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

 * 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 current Doctor and one of his past selves crashed together. (DW: Time Crash)
 * At the end of Turn Left shortly after Donna asks The Doctor what "Bad Wolf" means. The console room is bathed in red light and the Cloister Bell begins ringing, while the Doctor, answering Donna's question, announces that it is the end of the universe.

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)