Someone on Twitter is looking at writing code to create this Ghost toy sound.
I thought it was interesting so I recreated it in Ableton live to help with his research.
The algorithm for the 4 oscillators are D>C>B>A
Oscillator B and C have the envelope with a slightly slow curve as in the screen shot above.
The other Oscillators are no attack and just sustain.
the fine tuning is slightly out of tune to add to the wobbling tone.
The fact that FM synthesis is used has allowed layers of tones that interact with each other.
Sine waves were used apart from oscillator B which has added harmonics to make a spiky square wave. B is going into A
D and C are going into B.
Amplitude (Ring Mod) modulation is important to make the sound pulsate in its volume.
the mix of this and Vibrato gives the spooky effect.
The vibrato is from an LFO set to a fast rate but only a small amount used.
Bit Reduction, Saturation and making the sound mono took made the sound more "rough" in texture.
Last on the chain is a low pass filter to take away the harsher, digital sounding frequencies.
Legato and a slight midi pitch bend automation on the higer note is used.
The sound is mainly the synergy of the vibrato and the amplitude modulation
used on the right tone of oscillators from FM synthesis and the legato adds to the swooping nature needed.
The result in Ableton: