If it is a roaring sound it may be a fan clutch that is locking up early. When the noise occurs pull off the road and put the car in park, rev the engine and see if the noise is present. If yes see if it seems a though the fan is pushing a lot of air. Stay clear of the fan blades. I don't think it is exhaust or transmission because they would make the noise all of the time.

If you do replace the fan clutch, research which brand to purchase. Members recommend one brand in particular, I think it is Sachs???

Good luck and keep us posted!!!

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