‘ And the Angel of Yahweh said to him, “Why do you ask after my name, seeing it is Wonderful?” '
This may mean that His name was ‘beyond knowing', because He was beyond knowing in His essence, absolutely and supremely wonderful, or it may mean that it was a secret not revealed to men because it was too high for them, or it may be indicating what he was, ‘Wonderful', precious and important beyond measure. Compare the name given to the coming Messianic king in Isaiah 9:6.
Both Jacob (Genesis 32:27) and Moses (Exodus 3:13) had earlier sought to know God's name. Like Manoah Jacob was asked why he wanted to know it. To Moses was revealed the full significance of the name Yahweh, that He was what He would be. But both through it had life-changing experiences. It is doubtful that any of them sought His name to give them power over Him (although in pagan circles that was often the purpose of finding a god's name), rather it was that they may duly honour Him. Manoah certainly did not have the former in mind. He thought he was speaking to a prophet. But the words would undoubtedly stir questions in his mind.