When the car gest up to 40-50 mph it sounds like it should be shifting but doesn't. What could be the cause of this?

[christy77777]

I have a 2005 Saturn ion 3. I've noticed the last week that the car sounds fine until I get up to 30 miles an hour, it gets louder at 40 and when it's up to 50 it sounds like it should be shifting but doesn't. I only have 38,910 miles on the car. What could be the cause of this?