MCS Difficult to start

My MCS can be very hard to start, particularly after being left a few hours.
I know it isn't battery, its been tested and also cranks for ages. I have also ruled out ignition because it sparks fine even though wont start. once it does fire, it runs fine.

Now then; what I have discovered is: when I turn the ignition on it makes a buzz from under the bonnet for 2 or 3 seconds. If it makes this sound it will start. If it doesn't buzz then it won't start. I now know what to listen for before trying to crank it.

Does anyone know what this buzz is, fuel pump, relay pulling in, I'm not sure.

Thanks;
Kev