I know you said you dont think you have fisher cats there, but it sure sounds to me as if that is what is making the noise...and they DO sound really creepy!!
When/if you do discover what it is, make sure to let us know!
Bobcats have a sound that is similar to that and its creepy sounding to I've only heard one hunting and thats enough for me its creepy in the dark all alone in the middle of the woods!
What about toads? I know some of you may laugh, but the toads around here make a very creepy mating call. Used to creep me out until I figured out what it was. Sounds like it's coming from a much bigger animal.