My boyfriends truck was making this weird noise almost like a waterfall but 10 times louder!?

Question

At first it started when he would get about 50 mph and the let off the gas. We took it to the dealership that he bought the truck from and they said everything was fine but just recently it started making the noise everytime he let off the gas weither it was 50 mph or 20 mph. I think it might be something with his transmission but im not sure can someone please answer my questions i wanna get it fixed before something bad really happens!

Answer 1

could be the clutch fan engaging and disengaging.. could be normal.. depends though i really could'nt tell unless im there..

Answer 2

Take it back again and go for a test ride withthe tech. I have never heard of a "waterfall" noise in the 18 years I have been working on vehicles.