My 4x4 1.2 does this too. Not always. But when it does, the sound starts at about 1500 rpm, and doesn't seem to vary in speed as the revs rise after that.
I had to have a fault code read the other day (engine warning came on and car went to limp mode-- turned out to be a spark plug lead that hadn't fully reseated after my DIY service). But I wanted to know what the code was, just in case. The code (don't know what 'number' it was) was for a fault with the Evap(oration) Purge Valve, apparently. This is a valve by the throttle body that opens to create a slight vacuum in the fuel tank to prevent fumes escaping when the car is parked -- all part of the 'greening' of modern cars. The ticking noise is this valve operating too frequently, apparently -- either the valve itself is at fault, or the sensor wiring (which runs all the way back to the fuel tank), or the vacuum hose (a bright blue tube connected to the valve, which is at the back of the inlet manifold and goes right back to the fuel tank), or the pressure sensor at the tank end, or even a loosely-fitting fuel filler cap (which can prevent a vacuum being formed). Or at least, that's what the Bosch centre tell me. Can result in a slight loss of power. I've not fixed it yet...
[later edit: strictly, the vacuum is applied to the evaporation purge tank, which is connected to the fuel tank and I believe contains a charcoal canister to absorb escaping fumes too. I believe this is behind the plastic panel in the right rear wheel arch]