Loud Jackhammer like noise when accelerating

I have a 99 Chevy Astro that is generating a very loud jackhammer like noise when I am accelerating on the highway - such as when entering the freeway or passing. It is inconsistent as to whether it will happen or not every time, but when it does, its always when accelerating and reaching highway speed (60-70mph). If I let up on the gas, it dies down and goes away once its down to about 50-60 mph. It sounds like its right below the hood nearer the driver side. Sounds as though the cooling fan is rapping on a piece of metal, but I can't see anything near the fan that might do that. Can't repeat the problem if I am parked and doing high revs - only when vehicle is moving. When it happens I almost expect something to be hitting on the inside of the hood, but there is no damage there so can't really tell what is causing the noise.