If you mean at motorway speeds the engine is of course being additionally cooled by the massive airflow and is operating at relatively low revs for the comparative road speeds so I’d be surprised if you can even hear the electric fan at 60-70mph so it sounds like the water temperature is getting hot, the fan kicks in and the obvious drop in temperature, long journeys at moderate speeds so less air cooling is when you may notice it which means it’s operating as it should, same applies for lots of town stop start driving, this is when you should notice it the most bearing in mind weather the hot weather ambient temperature will affect the cooling system of the car, that seems like normal behaviour to me, anyone care to expand?