1997 Grand Marquis noise under dash

Hello,

my wife has a 1997 Mercury Grand Marquis. For the past year whenever the key is turned on she gets a clunking sound that comes from under the dash. I've had the radio out, the glove box, been under the hood and still cannot figure out what is causing the noise. It is driving her and me nuts. Turning the fan on or off makes no difference. I am at my wits end.
Any help out there?