What would cause a clicking in the dash of a car?

You really don't give enough information. Does this happen when the fan is turned on for heat?? If so, something might have fallen down the holes where the defroster air comes out of the dash and has made it's way down to the fan cage.

If it happens all the time, something could have gotten out of the glove box and is in the dash rolling around.

Sometimes a bad electrical relay will click when it is trying to make it's electrical connection and it does not have enough power or it's contacts are worn out.