vibrating noise from front end

My '95 Jetta has recently deveoped a noise that seems to be coming from the front of the car somewhere. It is like a vibrating noise but there is no obvious loose part, and the car drives smooth. All seems to be fine except that noise that is noticeable starting at around 30 mph. Has anyone experienced this, and if so what was the solution to the problem? FYI, it is an automatic.