My sister never spent much time in artillery observation posts, so what she reported as "that whistling thing; that sounds like a mortar bomb in flight" is actually the sound of an artillery simulator or "arty sim."

Mortar bombs (from mortars in Canadian service) are fin-stabilized, and produce little if any noise until "the earth-shattering KABOOM." The whistle is intended to represent the sound of the driving band and to cue those in the area where it lands to take cover. Driving bands, the bronze rings around artillery projectiles that engage the rifling, actually produce a fizzing sound from their spinning in the airstream as they pass overhead. If the round lands in your immediate area, i.e. within its lethal radius, you will not likely have heard anything before "the earth-shattering KABOOM."

Arty sims are dangerous pyrotechnics that can produce serious injuries, far more serious than most commercially available fireworks.