Ok, this one is weird.
I have a hissing sound in my dash.
When I turn the vehicle on it starts. My radio is off, my cooling fan is off-- heater/ac fan.
It changes sound intensity when I accelerate, or decelerate. When I get to freeway speeds-- main highway 3/10 mile from home--- it goes almost silent, but is still audible.
My initial question is-- does the 2001 have any pneumatically controlled systems behind the dash?
I'm wondering if a hose busted, or came off, has a leak, etc......
Thank you in advance.

 

sounds to me like you have a loose vacume line, probably under the hood, take a look and see if there is anything loose, to my knowledge there isnt any vacum controlled systems in the dash on the 2001+ trucks

 

Check EVAP lines near firewall and lines near brake booster.

If you hear it in the cab I'm willing to bet it's by the EVAP service port on the firewall driver side.

 

Ok,
It sounds like it's directly behind the radio, or the center part of the dash.

 

Use carb cleaner on all of the vacuum lines on the D. If the idle jumps up you just found a vacuum leak. I am not sure of vacuum controls on the 01 and newer D's but on my 00' I have a vacuum line for the HVAC box that goes in at the heater core.

 

Ok, I've looked under the hood and crawled up on the bumper, got my head in there, and listened.
I stood there for a few minutes, got down, walked around, listened to see if I could "triangulate" the position of the noise, and got back up in there. It's almost like it's in the black blower looking module that the air-intake goes into. but if it is, it's towards the back of it. As my rig has a skid plate, I don't see where I can look up in there without removing it, from the bottom.


Something else I noticed that it appears to be missing what look like two head bolts. I.e., mid engine, just in from what appears to be the electronic fuel injectors, there are bolt holes on both sides of the block.
Ok, it does not appear that I can attach photos. I've taken 3 pics of my engine compartment, showing the bolt holes, and the black blower looking unit..... So much for that idea.

 

Upload the pic's to www.photobucket.com and add the IMG link here.

 

I'd do what mean green says also spray some carb cleaner in there and see if it makes the engine rev a little, that will pinpoint your vacuum leak assuming there is one.
Also what engine are we talking about here?

 

This may sound silly, but if the airhat isn't sealed on the throttle body it will make a hissing sound that will increase with RPM's. and it can be clearly heard inside the cab.

 

True, it could also be the crank case breather hose not hooked up to the back of the air hat as well.

Not that I know of for the 2001. The older ones 1998-1999 (and possibly 2000 as well) do have pneumatically controlled heater actuators behind the dash. In the 2001-2003 they are all electric actuators.