Q: Car whines between 30 and 70 mph

Below 30 mph and over 70 mph, the car drives great, but not between 30-70 mph. When driving between that 30-70 range, when I give it gas my car performs well. But when I maintain a certain speed, there is a loud whining noise that sounds like the car is desperate for gas. The noise gets less when using cruise control, but is still there. I had an oil leak fixed at a local transmission shop and they hooked up the car to update all of the necessary codes. The problem still exists. A Jaquar specialist said that he had never experienced this problem before. In my research, I found another person who had the same problem and he couldn’t find a solution either. Can you give me any idea what is causing this?