1998 Amigo noise problem?

My Amigo make a constant noise as if I need to shift; however, I do not. When I shift into 5th gear around 30 miles per hour to 45 miles per hour it does not make as drastic a noise. The noise is louder than normal when I need to shift. It's like the engine is reving, but the RPM's are normal. What could this be?

Thanks for any imput....