clicking noise

I have a 1996 Mustang with about 80,000 miles on it. When I start the engine, especially in the morning, it makes a clicking/tapping noise that subsides after a few minutes. I checked the oil and it is not low. I can't take it in to be looked at until next week. Is this a very serious problem? Is it bad to drive long distances while it is in this state, or is it only bad while it is making the noise? Any help would be greatly appreciated, thanks!