Its from the coolant system!!! :P

Chances are its from the thermostat, thermo fan switch or one of the coolant temp sensors.

Just make sure they're all nice and tight. Maybe run the engine with the bonnet up and watch the engine as it warms up just to see where its coming from??

If its only a small leak it could be a small gasket or even a clip of some sort. You could probably fix it with some gasket sealant.