Gerade als er sich umwandte, vernahm er erneut ein Geräusch.
To me what this looks like is
Gerade - i imagine used as a particle and then a Temporalsatz:
als er sich umwandte.
I've read online about particles being non sentence members, that they can't sit alone before the verb bracket (Vorfeld), and that removing them doesn't change the meaning of the sentence (as you could tell from removing it here but i want to know the grammatical explanation to title question). But what's the explanation in this case here? It is a matter of a particle + subjunction to begin with, right?