...why you sometimes hear bells during Mass?

There are actually a number of reasons why bells can be rung during Mass. The first is to call people to worship, but it also lets people know about the calling down of the Holy Spirit and the consecration that is to come.  The bells can also be rung as the host and chalice are displayed, and lets people know that transubstantiation have taken place and that the Body and Blood are truly present. And not least, they make a joyful noise!
So now you know!