My mother had unlocked her phone (by accident) with her finger. So my brother realized that, whereas my mother did not. He told our mother that she had unlocked her phone and when our mom didn't believe it, he said:
There was an unlocking sound as well.
Is the use of "there was an unlocking sound" natural?
I would probably say "It made the unlocking sound."
If he hadn't presented any evidence before saying this, the "as well" seems kind of strange.
In addition using active voice, and specifying that the sound came from the phone by using "it" both improve the sentence in context.
Your sentence definitely seems too formal for talking about cell phones between family members.
'Unlocking sound' can be understood as a sound that caused unlocking. Also: 'as well' as what?
He could say:
But I could clearly hear the sound of unlocking (the phone).
