At least originally, keys scraped on a piano
According to the person who made it:
On how the Tardis should sound, he said: "I don't know who thought of
it, but we came up with the 'rending of the fabric of time and space'.
I was in a cinema and in the interval I had a programme and I drew it
- exactly how I wanted it to go together.
"I'd done a programme called The Survivors where we had to have the
sound of a ship scraping on the rocks, and the piano sounds I'd used
for that, very slowed down, seemed a good starting point.
Mr Hodgson used a device known as a ring modulator to create the sound
of the Daleks
"I got my bunch of keys out, I got my mum's front door key and scraped
that up the strings. We did that several times on the bass strings on
an old Sunday school piano that had been taken apart. "So we took
those and speeded them up, slowed them down and cut several of them
together and started to add feedback to get that echoey sort of
The takeoff sound specifically:
"They came to listen to it and said they liked it, but there was
something missing - why hadn't I put a rising note in it? "I said
'time machines don't go up, they go everywhere'. They said 'well we
think it needs it'. So I put the rising note in it with loads of
feedback and the Tardis was born.
The landing sound was the same as the takeoff sound, but with a "bang" at the end!
"Unfortunately, I'd spent so much time on the sound of it taking off,
when we were asked for it to land I only had three days to sort that
out. So I literally played it backwards, again with loads of feedback
on it, and put a great big bang on the end of it."