Elf Whistle
Take a piece of Elder wood - any length any diam and remove pith for at least 5cm. About 1.5 cm in cut a notch with a verticle face on the side nearest the end you removed the pith from and a 45 degree face on the other side. the notch should just meet the hollow where the pith was. Cut a piece of green wood as long as rg wood between the mouth end and the start of the notch which is the same diam as the ex-pith tube. Slice aprox 1/3 of the wood away down one side of this plug. Fit plug into tube so it is flush with the vertical face of the notch and the gap is aligned with the 45 degree face. Blow. adjust cuts until it whistles. Adjust tone by varying width of the bore/ depth of the tube.
I hope that helps!