Could be the water pump bearing colapsing or maybe the drive belt tensioner
Get the cam cover off and check as if it is the pump and it gives up it could take your cambelt with it
That sounds really scary!! Could this be why the coolant level indicator on the instrument panel has occasionaly lit up even though there seems to be plenty of coolant? Or is the sensor is on the exapansion tank and that it is just coincidence? I have noticed the radiator fan seems to kick in a lot without really thrashing the car even when just stood in traffic. Any idea how much a water pump would cost fitted? I was worried that it could be the engine bearings (ouch!) I am really hoping that it is just the water pump.
Do you get this noise when the car is stopped with engine on ? do you get a light vibration in the steering wheel even when stopping ? I got the same symptoms and it was a worn silent bloc that made the engine touch the chassis at one point...