Not to let a good "black helicopter" story get out of hand, but it was probably mosquito spraying. I don't recall what time of year, but Guard/Reserve C-130's used to do it in Virginia every year. Several passes at treetop level right around dusk. It leaves a lasting impression if it happens to sneak in from behind you at a couple hundred feet...
I was no more than 5 years old when I saw my first C-130 mosquito sprayer (Hampton, VA). I ran in the house and hid under the sofa, I was so scared. Now I'm in the flight line for the Navy base in Norfolk, and I get nervous if I don't hear them flying low at least once a day.