It is a thunderous sound that is both constant and very strong. It had to be activated by a person by generating friction between the floor and the chair when dragging it.
This is a smoothly rising shepard tone that i created using adobe audition 3's tone generator. I generated the tone as sine waves in 7 different octaves and mixed the results together. The period of repetition is 10 seconds. (this is a second version in which i removed the phase variance, making the sound monophonic for all practical purposes).
Generated sound in audacity as sinusoid 558hz - 0,8db for 5 sec. 2 effects applied on the sound: decreasing the speed at the beginningand changing the amplification at the end.
An augmented fifth in just temperment. The first tone is a sine wave of 300Hz, the second is 468.75 Hz, the third is the interval. Made with NCH tone generator