TARDIS engine

TARDISes were equipped with engines which were the source of the distinctive sound heard upon materialisation. By turning the engines "on silent", TARDIS pilots had the option of forgoing the sound, usually as a means of being discreet. The Doctor employed this function on rare occasions. (TV: The Impossible Astronaut, The Time of the Doctor) According to the Eleventh Doctor, the engine would shut down if no one was in a TARDIS. (TV: The Pandorica Opens)

Shortly following his regeneration, the Tenth Doctor "open[ed] up" the engines as he took the TARDIS through the Time Vortex on a course to the Powell Estate on 24 December, 2006, resulting in a crash landing. (TV: Children in Need Special)

Materialising the TARDIS in the Oval Office, the Doctor turned the engines on silent to be discreet. (TV: The Impossible Astronaut)

Taking the TARDIS to the source of the mysterious message beamed from Trenzalore, the Doctor turned the engines on silent in order to avoid "mak[ing] a fuss." (TV: The Time of the Doctor)

When attempting to diagnose the problem with the TARDIS following its dematerialisation from Skaro, the First Doctor and Ian Chesterton visited the TARDIS engine and power rooms. (PROSE: The Edge of Destruction) When the Doctor later attempted a temporal slingshot to return Ian and Barbara to Earth, the attempt blew the engine, the TARDIS needing days to repair it. (AUDIO: The Invention of Death) As the TARDIS was a space-time vessel, its engine was vulnerable to the energies of the Tick-Tock World, only managing to break free of the planet when the engine was augmented by the power of the TARDIS force field. (AUDIO: Tick-Tock World)

In an alternate timeline, the Eleventh Doctor had turned off his TARDIS' shield oscillators, leaving it vulnerable to the magno-grab of the Van Baalen Bros. salvage ship. The impact caused the TARDIS' engine to explode, only for the ship's safety protocols to freeze the explosion in a time stasis field. This timeline was later undone when the Doctor gave his past self the remote to shut down the magno-grab. (TV: Journey to the Centre of the TARDIS)