Cloister Bell

The Cloister Bell is an alarm bell located in the Cloister Room inside a 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)

The Doctor's TARDIS

 * The Bell sounded as a harbinger of the Doctor's impending regeneration. (DW: Logopolis)
 * The Bell sounded when the Master opened the Eye of Harmony in an attempt to take the Eighth Doctor's remaining regeneration, signifying that the TARDIS itself was dying. (DW: Doctor Who: The TV Movie)
 * The Bell sounded as the Doctor began to fly the TARDIS erratically (DW: Children in Need Special).
 * The Cloister Bell sounded continually went off when the Master cannibalized the Doctor's TARDIS into a Paradox Machine. (DW: The Sound of Drums)
 * It also sounded when the TARDIS of the current Doctor and one of his past selves crashed together. (DW: Time Crash)
 * It sounds 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. (this is interesting because in "The Doctor Dances", he clearly states that the sign for danger is mauve)

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)