The hollow stems of the Elderberry plant can easily be made into whistles, but it is not a good idea. Why?
Elderberry plants contain cyanide-producing glycosides. The cyanide is bonded to a sugar molecule, making it harmless to the plant. When the plant is injured, enzymes remove the sugar, letting the cyanide discourage creatures that want to eat the plant. There is not enough to be deadly, unless you ingest a LOT, but it can make you sick. Many other plants use similar glycosides for protection.