895.8

So I'm guessing the fan is suppose to be that loud

FireRescue4465

like i said, it sounds like you have a manifold leak

mud-digger

The fans in these trucks can be extremely loud. Not saying it couldn't be a manifold leak but it should be a shuttered sound like almost intermittent and he should definitely be able to hear it under the Hood or under the truck.

895.8

It's not a manifold leak they were replaced no to long ago

mr7confused

Get under there with a piece of hose or mechanics stethoscope and prod around for sounds, dont get yourself caught in moving parts