After stopping the engine following a 20-mile journey, I noticed a whirring sound from under the bonnet. Closer inspection revealed that this noise seemed to be from the upper part of the engine, forward of the air filter location. It was accompanied by the sound of water trickling into the coolant reservoir - perhaps overflow?
This eventually stopped after a few minutes.
Engine coolant temperature as indicated was normal, so I don't think this was an overheating issue. Does anyone out there have any experience of this "symptom"? Would it be advisable to have this checked by my local dealership?