Odola
Well-Known Member
The word "grind" makes me thinks of the ancient proverb "the mills of the gods grind slowly(, but they grind exceedingly fine.)" There must have been an equivalent for that proverb known to the Ringwraiths.
en.m.wikipedia.org
If so, the grinding sound itself is the message, the boulders are just there to make it.

Mills of God - Wikipedia
If so, the grinding sound itself is the message, the boulders are just there to make it.