As it could be either of these problems try to get a second or thiri opinion. I as well own a 1997 Explorer and found it to be the wheel bearings, but as well found that original equipment manufacturered transmission is said to be "Not very good" by a local transmission shop.
One of your wheel bearings are bad
Bearings misassembled, tire nosie, brake noise, noise in axle, noise in transmission, etc.
no--bearings and tires are the most likely cause of roaring--esp if it gets louder as you roll faster
That is your bearings. You need to replace them.
check front axle bearings.
Noises have to be heard to correectly diagnoise. It could be bearings, tires or a number of things.
Could be clutch cooling fan engaging
dirty exhaust system. All Matiz are like that. Could as well be rear weel bearings
bearings in hub usually goes on inside wheel first.new hub assambley needed
Check the wheel bearings and the front brakes for wear. These are the most common places that cause the roar.
Counter shaft bearing has failed inside the transmission.
when installing them always pack the bearings with grease, if the bearing has been run without grease and making a roaring sound it is probably bad and needs replaced