Vocoder
Lots of vocoder.
CHIKACHIKACHIKACHIKA.
Anyways, the reason it's so eerie is because you have two of the same patterns being played at the same time. One is the standard audio with a fuckload of vocoder, and the other is the same audio about two octaves lower and a semitone off, giving it that demonic sound.
And then you have the fucking grudge "UHHUGHGUHUGH" halfway in with the heartbeat, so that doesn't help.