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)

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)