94 Chevy k1500 Tranny Problems

The transmission in my chevy makes a whining noise that get louder with higher rpm's. It shifts late too. Even when I start it up and leave it in park cold or hot it makes the sound just idling. But this evening I started it up and it was was quiet. All I could hear was the engine and the exhaust none of that whining sound. Aamco said it need rebuilt completely but i don't know if i should believe that just yet. anybody know whats goin on?