VickyW, my girls are not normally fazed with a helicopter just flying over, it's that there is forestry just opposite my land (about 250m away) and recently the Police and Fire service have been hovering really low trying to catch the culprits who set the fires. It's been bedlam - they're even setting them at night. This wet weather we've had recently has been a Godsend, but with the next half term approaching and another dry spell of weather on the cards it'll be like living in the pits of hell again.
Lindeggs is right, they'll adapt to any regular loud noises, it's just the unusual events that throw them out.