85 300d that runs fine unitl 45 min to 1 hour at highway speed, then emits a slight chirping sound from the front (like a fan belt, but they check good) and a rumbling underneath that sounds and feels like exhaust noise/pressure. The rumbling increases in intensity when gas pedal is disengaged and the noise rumbles on all the way to a full stop (toll booth), at which point the car fixes itself and drives without problem. The only way to fix it is to come to a full stop, slowing down does not help. I can't take it to a shop unless I can find a mechanic that wants to ride with me for an hour. Can anyone offer any advise?