[Jim Koehler]

Also....
Check to see if the cone edge hasn't become unglued from the steel frame. The dust cover that sits UNDER the cone could have also become unglued. Does it have an apex spider support? Check to see if that's broken. Check the flexible wires to the cone...are they spaced away from it? If you press down on the cone, do you hear a scratching noise?.....could be dirt in the air gap. (use gentle air when blowing the dirt out.) Check for warping on the cone itself....that'll throw the concentricity of the air gap out of whack. (sp?)