Driving my truck when it sounds like I,m in a hail storm but there is clear skies

So what do you see hitting it as you drive?

nothing just the noise

You description of the noise indicates the sound is coming from the body. May want to find someone who knows a bit about vehicles and take them for a ride to pinpoint the noise. You could have something loose under the vehicle like the exhaust system.

It doesn,t happen all the time - it does totally sound like the body. But everytime the ac has been on.

It is possible the A/C compressor is going bad or there is just a bad clutch bearing that could be replaced. Have someone listen to it.

Can a ford garage plug the truck into the computer and will it show up

Don't know what year truck you have, but no, noise does not throw a code. By the sounds of this I believe someone with an experienced ear can tell you what the problem is. Just stop by a shop an have someone test drive it to see what they say. if you want then post it back here.