You truly can find tools for conjure in the strangest of places. I picked up this rattle made in Kenya at the zoo, of all places. All you have to do is bless it and consecrate it and it's good to go. This will be my back up rattle as my primary rattle is one of my own creation.
The rattle as a tool is used for the calling and communicating with spirits. When employed it serves as a sign or signal to the spirits that the individual is seeking communication. The attention of spirits can be captured this was just as the attention of the living can be.
One doesn't have to use a rattle. Some workers may prefer bells. Some may prefer pounding sticks. Some may prefer tapping a horn on the altar or tapping a staff on the ground. Some may forego tools of such nature and just clap their hands or shout to get the spirits attention. It's really up to the preference of the individual.