Memory's not what it used to be, and it's been awhile, but don't those cone covers use a brass or bronze spacer/shim about 1/8" thick? They come in varying thickness', as all cones are not created equal. ie, polished alum and chrome covers, the chrome needs a thicker one. Like I said it's been awhile and I may be thinking of an even older model, and be wrong all together, but I thought it might be worth mentioning. If you changed covers and not the spacer, that might be the reason for the noise. Might be worth looking into.
Nate