Cloister Bell
From Tardis Wiki, the free Doctor Who reference
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 It went off when the Master cannibalized The Doctor's TARDIS into a Paradox Machine. (DW: The Sound of Drums)