thumping noise

I have a 2004 malibu maxx with 65,000 miles. Between 40-50 mph I hear a thumping noise under the car, especially when turning to the right. It seems to go away when going straight or toward the left. My usual mechanic thinks it is the rack and pinion steering but while at the dealer last week, they said they think it is a wheel bearing. Which would you think and how can I find out before I pay to have the wrong one fixed? thanks!