This is a stupendously fun sound, and definitely one of my favorites!

To make the sound

1 - Learn the siren sound, both inwards and outwards
2 - Do the siren outwards, then inwards
3 - Do this quickly and adjust the pitch to get the clown horn sound - the outward burst should be high pitched, the inward even higher!

Peace, love, and we all float down here, Georgie

-FT