Rattles are used extensively in contemporary Shamanism...as "noise-makers".... a rattle is easily held in the hand, and can be used easily while dancing, chanting and invoking power, while moving about or sitting down. ... drums are the Shaman's noise makers par excellence, but rattles are light, can be used in situations when noise is needed but shouldn’t perhaps be shared too widely, and are tough and robust. 

Shamans use sound to enter trance states for healing and accessing the Otherworlds, and for causing shifts and changes in their own and other peoples' energy fields. Sound has powerful effects on the body, the brain and the mind, and the sounds made by rattles are said by native peoples around the world to attract the attention of helpful Spirits. The sound from a rattle is perfect for all these needs, and has the benefit of not travelling too far- it won’t disturb your neighbors at home, or bring unwanted attention outdoors. 

Rattles are also used for cleansing energy fields...and are used for this purpose at the beginning of ceremonies and rituals to ensure there is no negativity in that place... Rattles have another quality:  their form lends itself well to decoration - carving or beadwork or windings on the handle, artwork etched on the ‘head’ of the rattle...they can be adorned with totem signs and symbols, and this will all help to keep these connections at the forefront of your consciousness when you are using your rattle.

From: http://www.galdraheim.kirion.net