I have found in certain kabbalistic documents that contain names of angels there is an instruction not to say the names of those angels aloud - example with names in parenthesis. I realize we are not allowed to say the name of God aloud the way it is written, but why are we not allowed to say the names of angels?
Rabbi Chaim Vital writes in Shaar Hamitzvos (Parshas Shemos) that the Arizal warned against pronouncing the names of angels aloud. He explains that this is because when the angel hears his name being called he is required to come down. When the angel sees that he was summoned unnecessarily he becomes upset and can accuse the person.
The Nachlas Yosef (end of chapter 9) writes that the five angels Michoel (thankfully!), Gavriel, Raphoel, Uriel and Nuriel are excluded from this, and people can be named after them.