Dumbledore's mysterious curse against Voldemort

Question

In Order of the Phoenix, at the start of Dumbledore's duel with Voldemort he casts a spell that seems to have no effect. What was that spell?

Dumbledore flicked his own wand: the force of the spell that emanated from it was such that Harry, though shielded by his golden guard, felt his hair stand on end as it passed and this time Voldemort was forced to conjure a shining silver shield out of thin air to deflect it. The spell, whatever it was, caused no visible damage to the shield, though a deep, gong-like note reverberated from it – an oddly chilling sound.

‘You do not seek to kill me, Dumbledore?’ called Voldemort, his scarlet eyes narrowed over the top of the shield. ‘Above such brutality, are you?’

Answer 1

Canon

We don't know. The spell isn't referenced anywhere else in the Harry potter books, nor have there been any pronouncements on the subject in any of the additional canon resources such as Pottermore or interviews with JKR.

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Guesswork

Voldemort explicitly confirms that the spell isn't lethal. Under the circumstances, we can presume that Dumbledore intended to immobilise Voldemort or otherwise incapacitate him.

In Harry Potter and the Half-Blood Prince we learn that Dumbledore can cast an especially potent Full Body-Bind Curse wordlessly. This seems as good a guess as any:

Harry’s body became instantly rigid and immobile, and he felt himself fall back against the Tower wall, propped like an unsteady statue, unable to move or speak. He could not understand how it had happened – Expelliarmus was not a Freezing Charm –

Then, by the light of the Mark, he saw Dumbledore’s wand flying in an arc over the edge of the ramparts and understood … Dumbledore had wordlessly immobilised Harry, and the second he had taken to perform the spell had cost him the chance of defending himself

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