Hey LMS, ours has done this since we got it; many thanks for finding this, so .......
Pop off horn thingy
Mark position of round steering things on its splines
Large socket, remove round steering thingy - at this point not only no grinding but no steering thingy either
Then clean and grease the brass rings
Put all back together, and this is where mine will get really interesting, try making big van move in correct direction using non grating round steering thingy
On the plus side, we went away this weekend, and the air vent thing in the front sounds as though it has angry bees inside it

(would make a good phone game? Angry bees?)