One of the more common problems with Ford motors of the mid-1980's is a pinging or knocking sound coming from the 5.0 engine. The main culprit seems to be the pistons, especially when the motor is cold. Seems there were some factory problems with flat top aluminum pistons for mid-1980's Ford engines. With aluminum pistons in a cast iron block, you're dealing with dissimilar metals. And each metal will expand with heat at a different rate. Cast iron is much more resistant to heat; aluminum responds quicker. So Ford undersized the pistons slightly to allow for this expansion. What it does is let the pistons slap around when the motor is cold. You will probably notice this more at a cold start than any other time. Usually the problem becomes much less noticeable once the car is warm and the pistons have expanded. If your engine also uses oil, that seems to be working hand-in-hand with this problem (see the section on Motor Uses Oil for more information).
The only permanent solution to get rid of the noise is to change the pistons, and as you can imagine, that's not fun, practical or cheap. You can try to use some motor treatments or thicker weight oil and see if that helps; some people report that it does. Or try running premium unleaded gas (93+ octane). Not all motors make the noise, so yours just might be one of the lucky ones. Otherwise you may just have to live with it. We've been assured by many Ford professional mechanics that this will not cause any damage to the cylinder walls or the cylinders themselves. Incidentally, when this problem was first detected in the late 1980's, Ford issued a "secret" recall on the engines, which meant that if a customer complained enough, Ford authorized their dealers to replace the entire engine at Ford's cost. Unfortunately that lasted only a few years, and you cannot get a new engine anymore, so the solution is up to you now.