I'm late and many good answers have said what I couldn't so:
I would experiment with an electric guitar and a piece of metal or something to create some good scratching sounds which to me communicate tension and something about to be pushed past the breaking point.
Maybe some extremely loud screaming vocals or screams turned down to be barely audible under the foley or real-world sounds can create a nice subliminal bed for what you want to communicate.
Pulsating low-end. Sort of like when the dreams start collapsing in Inception.
High pitched frequencies 12.5K and above ramped up to an exaggerated volume after dropping out all other sound (sort of like silence, but with a tone - think opening scene of Children of Men).
What about something totally out there, like a rattle-snake rattle or cicada?
Don't forget this advice:
“Sometimes in the creativity area like sound, my instincts tell me to do one thing, but I do exactly the opposite and see what happens. For example, if you’re designing a scary weird creature, try putting in a high-pitched, low-level sound. Who knows, maybe that will be scarier.” Gary Rydstrom
Hope I could help.. Good luck!