So about a week ago I was getting on the expressway and accelerated to merge with traffic and I heard a loud whistle maybe even a whining sound like something was spinning up and that's when I realize my GT wouldn't shift into 4th gear( or overdrive please correct me if i'm wrong) I can still drive the car as long as its below 45mph it runs fine but, when I need to go faster it feels like it wants to shift but doesn't. I'm hoping for any advice from anyone that may know what could be wrong with the transmission. I checked for error codes and also had them check the electrical to the transmission at a shop and everything was fine so I know its internal. The car has 156,000 miles on it.
98 Pontiac Grand Prix GT