I think that a 1 second blip should suffice and don't suppose that stalks do their stalking after dark. That may be a case of anthropomorphism not being a case of black and white (night and day)
What's the minimum duration (half second) to give an effective sound burst? A couple of bits of pipe and a servo to close off ports could have the blip appear at different ends of the plot. Movement in one place and sound from a range of areas. More than enough to unsettle your average heroine, even the two legged variety . . .
Shades of Close Encounters of the Third Kind.
The real secret with neighbours and the council is a few short blips at 1am, then make sure that for day 2, day 3 and day 4 that the device is powered by a Lipo battery and radio controlled. Placed in the bottom a hedge at various neighbour's property down the street. Not enough time to locate it, but enough to annoy. Skip day 5 and give them two blips on day six. then retire the device and hope that it hasn't been found. Place in hedges that are not in obvious need of clipping to reduce the chances of detection. Must put it on my project list . . .