I'm not sure of anything (and especially not in English).
The crow is perched on a gargoyle and it says "Caw... Caw..."
I don't know how else to do anything but whistle or imitate its cry.
I'm sorry but that is too obscure as it stands. Especially given the responses to things like TALK RAVEN and SAY HELP... If the raven responded with CAW? to those, then maybe I'd have got there.
I might have got WHISTLE in the end... I'd been trying virtually every other way of making a noise... but I think you're going to get players giving up in the tower without an extra hint. I completely get that the throne gives the hint that the raven is going to be the solution.
One thing you could do, is time how long the player has been stuck in the castle and change one of the examine messages, or add an extra environmental message, if they've been stuck there too many turns. Something like "The raven is out of reach. You wonder how you could get its attention."