Noticed about a month ago on my 2001 Deville base that when heater/ac running, fan slow or fast same vibration level that the steering wheel has vibration. I turn off heater vibration stops. Turn heater on vibration starts again. Obviously blower motor running. I haven't gone looking yet but is it possible that it has come loose? No codes relating to this just a vibration. I never noticed it before a month ago. Could something be loose? Any ideas?