An SF340 flight crew experienced a vibration on departure violent enough so as to warrant an emergency declaration followed by an uneventful landing at their departure airport.

Narrative

During our climb out at approximately 7000 FT MSL we noticed a non normal vibration with increasing intensity in the flight controls. The rudder seemed unaffected. The flight controls began vibrating so violently you could physically see and fell them moving. I would compare the vibration to almost like a continuous stick shaker. I turned off the autopilot to see if the auto pilot was the problem; but the issue still existed. All engine gauges and all other warnings in cockpit were normal. I decided to declare an emergency because I felt the airplane was far from a normal state. I had my First Officer check to see if we had a checklist for uncontrollable vibration in controls and there was nothing. The vibration continued until we selected Flaps 15 for the ILS Approach. While the flaps were moving to the 15 degree setting the vibration went away. We then continued on the profile for a two engine ILS and landed safely. After landing I advised ATC that the fire department was no longer needed. I then taxied to the gate without incident.