My 96 V8i with only 85K miles has a noisy viscous fan on start up and can take around 200 to 300 yards before it goes silent.

My Tdi's have never done this, so is my viscous coupling faulty?

When towing in sunny weather (Ha Ha not very often) the fan can be heard engaging especially in slow traffic.

Not an issue at the moment but will it need replacing?

Normal sounds like a turbine, unless it's a new sound that you have not heard before.