The oldest question, hidden in plain sight: "Doctor Who?"
This information was given by Dorium Maldovar to The Doctor. How did Dorium know about this? How did Dorium manage to get hold of this information, when even The Doctor and the Teselecta did not know about it?
The information Dorium gave was about the Silence, and their phrase "silence will fall" (or, as he says, perhaps more properly translated as "silence must fall").
Dorium knew that the Silence were trying to stop this question being answered, due to a prophecy that Bad Things would happen if it were answered.
We now know, after, The Time of the Doctor, that the Bad Thing was the return of the Time Lords to the universe, and the resumption of the Time War.
We never find out how Dorium learned about the Silence's motivation. But I don't think it's particularly surprising for Dorium to know this, and for the Doctor (or the Teselecta's crew) to be unaware of it. There's no reason the Doctor would know about the Silence's motivations, and every reason he wouldn't - you forget having seen them as soon as you look away from them, and their entire purpose is somewhat anti-Doctor, so they'd probably be keen to hide it from him.
Speculating, Dorium is a black marketeer (we see him selling a coveted vortex manipulator to River), so he's likely to come into contact with a lot of shady people, and thus hear things. Beyond that, he told the Doctor about the question after he had been beheaded by the headless monks, who (I think?) are part of the Silence. He may have overheard this detail from inside his little wooden box.
The Silence knew what the question was - because they were trying to stop it being answered. Their entire religion was based on the belief that this question "*Doctor who?" must never be answered.
It stands to reason that anyone who worked for The Silence could know this, or at least glean that knowledge from what they were trying to do - kill the Doctor before the end of his 11th incarnation.
When The Doctor asked for information on the Silence, he was directed to Gantok, who apparently worked for them. In turn, Gantok told The Doctor that Dorium Maldovar knew many things about his death.
So it would seem that certain persons involved with The Silence knew about the question mentioned in the prophecy and that Dorium was part of that chain.
Dorium was a trader of information and goods, he also claimed to have done business with the headless monks and their order. Its kind of hard to explain what you want without a head so from a logical point of view they would have to have someone in the order who still had their head make the deal and trade the information.
