weird sound between 45-50 mph

My g/f just bought a 2002 liberty renegade. When you are accelerating it makes a wierd sound when you get to 45 mph and it goes away when you get to 50 mph and it doesnt come back after that. The best way I can think of to describe the sound it makes is it kinda sounds like the sound you hear when your going across a bidge. To me it sounds like its coming from the rear tires but my g/f says the engine. When she first told me about it I thought it was the rpm's just getting high right before it changed gears but that doesnt seem to be the case. Does anyone else have this problem? She is going to take it back by the dealership but I have a feeling they are going to tell her its normal.

I haven't had this problem with my Liberty, but I have had the problem with cars I have owned in the past. Try having the wheels and tires balanced. I know that has fixed my problems in the past. Simple and fairly cheap fix.

Covman