The whistle is simply a startle (as you said it interupts his behavior), a hand clap, foot stomp or finger snap should accomplish it just as well. If he isn't listening to your voice correction then he isnt respecting your word. Be careful that he doesn't think you are just barking too.
He has to feel your energy come towards him, in order for him to understand what is being said. Your tone should be short, firm and abrupt. Make your point and then back away. Do not berate him or freak him out, simply make your point and then back away. It might take a few corrections for him to understand the consistency of his behavior and your response. When you see him calm down, you can calmly praise him.
Then hook him up on a leash and spend some time with him. Work the things he knows and teach him something new. A bored puppy is an obnoxious puppy.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended