1999 Dodge Stratus

Hi, my mom's 1999 Dodge Stratus, 130,000 miles, makes a roaring noise going down the road. You have to be going at least 45 MPH to hear it, it gets louder, the faster you go. One shop said it was a bad wheel bearing, they relaced that, it still does it. They also replaced the rotors and brake pads. Tire shop said it was the tires, bought new tires, had them balenced, it still does it. We have checked the exhaust, thats not it. You cant hear anything whe it is just sitting and idling, it has to be going down the highway, at least 45 MPH. My mom is on a fixed income and can't afford anymore unneeded repair bills. If anyone can help up diagnose this. It would be greatly appreciated! Someone told us it could be a valve in the transmission.