There is a preset called the Whistle Project for the PCM Synth in the preset downloads but its so old that Caustic 3 ® can't use it (says its made with an older version of Caustic ®).
It might be hard to correctly synthesize a human whistle but I'm sure there are people out there who have done a decent enough job. Also need a gym whistle and any other kind there is to offer if possible. I'll take anything. THANK YOU!