First, ditch the idea of a straight jet. Grab a cheap ducted fan from an old fan or a car radiator fan, splice it into a small metal tube you can pull from a bike frame. Wrap the whole thing in foam or even old carpet—just to muffle the hiss. Add a little exhaust pipe from an old lawn‑mower muffler, bend it so it folds back onto itself, like a snake, to make the sound bounce off itself instead of going straight out. Put a small turbocharger you can salvage off a motorcycle into the inlet so you get more pressure, but keep the fan spinning at a higher RPM than it would naturally; that pushes the air faster but the muffled duct keeps the noise down. Finally, if you really want to be reckless, bolt a spare piston ring around the whole assembly—just to make it feel like a real engine. Done, no regulations, just pure speed and a whisper of jet noise.