Why is it when i start my car after leaving it park for week i get some sounds coming from the engine fan-belt?

i noticed the noise goes away after the car has been left running for some time, i am now worried as someone told me i might have a problem with my valve setting while i strongly suspect its the engine fan-belt that is giving the sound, i say this because when i add some water to the belt when i freshly start the car the noise goes away but resumes shortly as soon as the water dries from the fan-belt. the sounds is sort of intermittent tapping that goes away when the engine runs for a while as soon as i let the engine run for up to 4-5 minutes.