I have NEVER heard a cyclist with a whistle and I would NEVER associate such a noise with one.

Use a bell or your voice. Both are the only things likely to be conditioned into another trail user as a polite cyclist wishing to pass. NEITHER is universally understood by all trail users, but better than nothing I suppose.

It is still always on you, the cyclist, to pass safely without hitting another trail user regardless of how oblivious they are being.