Why is my engine/timing belt ticking when driving?

My Vauxhall Corsa C 2006 model 1.2 Twinport timing belt has been replaced in time of 5 years that I have owned the car, yet it sounds like this is rattling/ticking loudly when driving. The noise comes and goes, and isn't associated with a certain gear or speed. It tends to get worse as the revs increase. I have noticed that when I fill the water levels up, so the radiator is cooler, the noise tends to get better.