If it's the sound I'm thinking of, I think it's a theremin.
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GusB
XAM2175
I think actually that the theremin is an instrument that can be used to replicate it, rather than being the original tool. Suggestion seems to be that precious few instruments (in the traditional sense) were used at all:
en.wikipedia.org
Doctor Who theme music - Wikipedia
en.wikipedia.org
hexagon789
I believe the very first version used a key scraped along a piano wire to produce that unique sound
XAM2175
@hexagon789 that was the effect for the TARDIS coming and going, IIRC.