When I open my bonnet I hear a slight buzzing noise which stops after about 10 -15 seconds. Can't trace exactly whereabouts in the engine it's coming from. 

I'm not really bothered about it - I'm sure there's a perfectly good reason for it. I'm just curious. Anyone else noticed this ??   

When you turn off an engine it still makes sure it cools it's self.

There is coolant being pumped.

6 minutes ago, Rooted said:

When you turn off an engine it still makes sure it cools it's self.

There is coolant being pumped.

The engine has been turned off for a least and hour - thought it would have cooled down by then ?  Or does it still go through the coolant pumping process regardless of engine temparature ??

That is different then. 

So when you open the drivers door when the car has been stopped a while it primes the fuel.

 

I take it you open the door to open the bonnet.  That will be the buzzing.

@john1952Have you had a chance to listen with the car locked, then unlock and open the drivers door and hear if the priming noise is what you hear under the bonnet?