I have a 2012 F350 and have a squeal during acceleration. It happens when is get in it hard and goes away. The dealer told me it was the belt tensioner, but it doesn't do it a at idle.

Thanks for the help in advance.

 

belt or tensioner would be my guess. How many miles are on the truck?
They make a spray that you can use on the belt to see if that helps, but my guess it won't.
It is either slipping because it is wore out or the tensioner.

Have you reached in there....with the motor off and keys in pocket, and checked the tensioner or visually watched it when the motor is running?

 

I would check the belt and associated path, also take a look at all the air boots, cracked rubber boot, or loose clamp can cause a shrill whistle under load sometimes, maybe even air filter box.

 

Apply a small amount of dielectric grease to both sides of the belt (engine off) and see if the noise goes away. If it's a high pitched squeal that gets louder and higher with rpm it's most likely an exhaust leak. Possible broken manifold studs or cracked egr pipes. Any soot spots under the hood?

 

I recently had a similar problem. It would squeal under load, but was really bad when exhaust braking. Turned out to be 2 broken studs on the Exhaust manifold Left side. Ford knows about this problem and makes better studs for the #4, and #8 cylinder. Not sure if this fits your noise or problem, but with a light and a mirror I could see where the exhaust was leaking prior to tearing it apart.