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.

When the Cloister Bell tolled, it sounded like a big 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)

It went off when the Master cannibalized The Doctor's TARDIS into a Paradox Machine. (DW: The Sound of Drums)

It also sounded when two TARDIS' crashed together. (DW: Time Crash)