2004 Ford Ranger Weird Front suspension noise

[tjr1000]

This problem has me baffled and it's hard to describe. At speeds of about 25 miles an hour on not very rough streets, I'm getting a noise that sounds something like a drum being hit in the front engine compartment. It doesn't do it at highway speeds or on really rough surfaces. Mechanics have looked at it and can't identify the problem. Anybody have any ideas?