Many siddurim include sections before tefillot and mitzvot (such as Baruch She'amar, tzitzit/tallit, and tefillin) kabbalistic declarations of intent, typically beginning with the words hareini or hareinu or the formula of l'shem yichud…. Is it appropriate to say these aloud? When davening before the amud?
The Shla (in mitzvat tefillin) wrote in the name of the Zohar (Emor) that "le-shem yichud Kudsha Brikh Hu..." should be said, as brought in the siddurim. This is agreed to by the Artzot Ha-chayim (25, Ha-me'ir La-aretz 29) and others (see, e.g., the introduction of the Shev Shematta).
In contrast, the Noda Bi-yehuda wrote (OC vol. II, 107 and YD vol. I, 93) that it should not be said before any mitzva.
So it depends on your Minhag/Nusach.
I have seen that in places where the Leshem Yichud is said during davening (e.g before Baruch Shaamar) that both the Mispallelim and the person davening before the amud say it out loud but do so in a softer/quieter voice than the level they daven the rest of the davening with.