There is a habit passed around some choirs here to sing a descending few notes – Do La Fa – to anchor the ear and find the right note. The sequence helps get away from the modern major scale and cue the brain for more modal sounds.
And it’s not just for the beginning of a piece, but often as a safety net in case of mishaps along the way.
So, if you are helping me out at Mass and we’re ready to begin the Communion Antiphon, hum a Do, La, Fa before giving me the starting note.
I don’t know where this practice comes from, but I find it very handy myself. I offer it here in case someone else finds it useful.