This item is a crotalus, but what is a crotalus? It is a large wooden clapper used on Holy Thursday in church services. 
 
The Holy Triduum is the shortest – but most important! – liturgical season of the year. Beginning with the liturgy on the evening of Holy Thursday, it lasts three days until Easter Sunday.
Due to some unique rules for the Triduum, if you attend Triduum liturgies, you may hear one of the rarest of liturgical instruments: the crotalus.
In the Roman Rite, altar bells are not supposed to be rung after the Gloria in the liturgy on the evening of Holy Thursday, and are supposed to remain unused until the Gloria on Holy Saturday. This is supposed to make things more somber as we remember the passion and death of our Lord Jesus Christ.
But, during this short period of time, is anything supposed to take its place? That’s where the crotalus comes in. The Church’s liturgical rubrics don’t prescribe a replacement for altar bells, but there is a long-standing tradition of using a wooden clapper or noise-maker in its place. This serves to both mark the same events as the altar bells, but in a less “sweet” way and thus maintain the somber tone.