So basically, it's like a high pitched shriek that slowly devolves into a more human sounding groan. It sounds like this:
https://www.dropbox.com/s/8mre0e7zy6ur9cq/voltigore_die1.wav?dl=0 (ignore the "voltigore_die 1" title, it was just the voltigore's death sound from sven co-op that I replaced with this sound).
It's heard in Back to the Future as part of the DeLorean's time travelling sounds. It is also heard in Ben 10, notably several times by the Limax in Episode 4, when the Mycelium is killed in Episode 20, and several times by the HeatBat in Episode 25, albeit higher pitched.
An edited version of this sound can be heard here: https://soundideas.sourceaudio.com/#!details?id=11530554
Does anybody know a clean, unedited version of this sound effect that I can find?