I've developed a rumble on my '03 stock Excursion with 69,000 miles on the odometer. It only happens between 55 and 68 MPH and only opon acceleration. When I let off the pedal at it goes away. It sounds like it is coming from up front either or toward the middle of the vehicle. Not only can I hear it but I can also feel it in the seat. Possible culprits, anyone? Thanks.