I believe it is roaring noises that you are hearing and nothing to do with the transmission or engine and these could be from the rear differentials, transfer case, tires or wheel bearings.
When the noise appears, does releasing the throttle pressure affect the noise level? If yes, it should be drive train related meaning it could be from the differential or transfer gears. Have you checked the oil level in both components?
If releasing the throttle or cruising in neutral does not affect the noise level, you should be looking at the wheel bearings and tire conditions.
It is impossible to understand noises and provide recommendations online and the mechanic who did the test drive should be in a better position to make any recommendations.
Friday, July 8th, 2011 AT 12:55 PM