Objection 1. It would seem that an angel does not speak to God. For speech makes known something to another. But an angel cannot make known anything to God, Who knows all things. Therefore an angel does not speak to God.
Objection 2. Further, to speak is to order the mental concept in reference to another, as was shown above (Article 1). But an angel ever orders his mental concept to God. So if an angel speaks to God, he ever speaks to God; which in some ways appears to be unreasonable, since an angel sometimes speaks to another angel. Therefore it seems that an angel never speaks to God.
On the contrary, It is written (Zechariah 1:12): "The angel of the Lord answered and said: O Lord of hosts, how long wilt Thou not have mercy on Jerusalem." Therefore an angel speaks to God.
I answer that, As was said above (1,2), the angel speaks by ordering his mental concept to something else. Now one thing is ordered to another in a twofold manner. In one way for the purpose of giving one thing to another, as in natural things the agent is ordered to the patient, and in human speech the teacher is ordered to the learner; and in this sense an angel in no way speaks to God either of what concerns the truth, or of whatever depends on the created will; because God is the principle and source of all truth and of all will. In another way one thing is ordered to another to receive something, as in natural things the passive is ordered to the agent, and in human speech the disciple to the master; and in this way an angel speaks to God, either by consulting the Divine will of what ought to be done, or by admiring the Divine excellence which he can never comprehend; thus Gregory says (Moral. ii) that "the angels speak to God, when by contemplating what is above themselves they rise to emotions of admiration."
Reply to Objection 1. Speech is not always for the purpose of making something known to another; but is sometimes finally ordered to the purpose of manifesting something to the speaker himself; as when the disciples ask instruction from the master.
Reply to Objection 2. The angels are ever speaking to God in the sense of praising and admiring Him and His works; but they speak to Him by consulting Him about what ought to be done whenever they have to perform any new work, concerning which they desire enlightenment.
Source: Summa Theologica (New Advent)