An irritating clicking noise

I have a 1988 Cultass Ciera International that I love. But every time I turn on my car I hear this clicking noise coming from what my father thinks is the alternator. When I press the accelerator the clicking noise goes away. However, when I go slower than approx. 15 mph the sound comes back. Also, when I stand idle, like at a light, the sound comes back. It runs good for it's age and we just got a tune up and oil change. I also have to mention that it happened after I loaned the car to my brother. He went too fast over the railroad tracks then the sound started. Or so he says. I would appreciate any feed back from ya'll.