Water pump failure
 

What would be the immediate symptoms of a water pump failure (fractured impeller). In particular, would there be any noise?

I would assume it would make considerable noise, especially if the broken impeller made contact with the housing internally. The excess movement of the fractured blade(s) against the coolant would probably emit some sort of muffled whirring noise that would be louder than normal.

I heard a seized water pump make a sort of high-pitched "chirping sound".

Typically when the coolant pump goes, it blows a seal, and the result is a slow leak through the weep hole. Can't see this from the coolant pump because of the tight confines, but the resultant coolant loss as evidenced by the pool directly underneath the car is a sure sign of coolant pump failure.

Another, rarer, failure mode is a loose impeller. The impeller shaft spins at the appropriate belt-driven rpm, but the impeller either doesn't spin at all, or at a much lower speed. I've seen this on Ford's Triton V8.

I have NEVER seen an impeller related problem with a Mercedes water pump. They usualy have a bad bearing, or just leak.