An altar or sanctus bell is rung during the Latin Rite of the Catholic Church, Lutheranism, in Methodism, and in the Anglican Communion. After a little research I found the reasons for ringing bell; first, to create a joyful noise to the Lord; second, the Church bells ringing signaled those not able to attend Mass that something supernatural was taking place. And, they have been a part of the Mass for about 800 years. If you look closely you will notice the bells are different sizes and therefore give different tones when rung. The gentle ringing of these bells is really not disturbing but I suppose could be startling to those who are not expecting it.
My roots are from a Protestant childhood. I converted to Catholicism in my mid thirties, at a time when the church provided much needed support and growth. I call myself a Christian but with reservations with the actions of some professed Christians. I seldom attend Mass anymore for personal reasons, yet I do search out churches where quiet can be found. Just as in nature it is here that I can read, write, spend time in pray, meditation and contemplation. Finding these places helps keep me connected to the Divine and sometimes learn about sanctus bells.