A character is, for example, standing on a cliff's edge when they are startled by a helicopter which rises, roaring, into view. It didn't make any noise until someone saw it. This happened on Thelma and Louise and about a dozen other more forgettable movies, like Charlie's Angels.
The original working title was Surprise Helicopter, but that limits the aircraft to just helicopters and now I can more easily put this in Vehicle Tropes
I don't know how many times I've seen people get hit by a car that didn't make any noise until it hit them, for example: the second(?) episode of The Sarah Connor Chronicles
Also, I remembered the Humongous Mecha
from Terminator Salvation that was able to sneak up on people despite its earthquaking footsteps and the deafening Scare Chord
noises that its gears made every time it moved.