Did you see it? I can offer several suggestions as to what it may be.
First, secret government air craft... The USAF has been experimenting with high velocity pulse jets for about 30 or 40 years now. Some people
suggest the Aurora spy plane uses one of these high output pulse jets. They sound like helicopter rotors, but they rip across the sky. They sound
REALLY cool. I've seen a couple of these test planes pass overhead and they don't sound like anything else on the planet! Really cool stuff.
Next up, it could have been a Gluhareff Pressure Jet. They are similar to pulsejets, and sound the same. E.M. Gluhareff developed it, and tried to
patent a Backpack helicopter. I don't know how successfull it was, but if someone was testing one in your area, then perhaps it is what you
Other than that, maybe it was just a helicopter moving really fast...