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 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)

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)