Diesel backfire?

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?

Thanks,
Clay

[Robby Ackerman]

I'd check the rear end and look and see if the axle half shafts are loose.

Thanks, I will check the rear. I also have noticed a miss at idle and that lead me to check the injector lines. I found one wet with diesel and will replace it today if I can find the right line. I still don't understand how it would wait 45 minutes to give me fits but I'm no mechanic.

Clay