with the engine running spray WD40 or s similar water despersant (not oil) onto the belt on the drivers side of the engine, thats probably the one doing it, it should stop, may come back and it will need doing again.
But whatever you do dont spray anything on the cam belt, not that you can anyway without fiddling.
It could be something else but its a usual target for noises, you should be able to see it down betwenn the engine and other gubbins.