I noticed a few days ago a hiss when I turn my defrost on. The noise stays for a few seconds after I shut the truck off, then dies down. When I 1st noticed the hiss, it was followed by a leak in the radiator (next to the cap). I pulled a radiator out of a 97 and replaced them. While filling the radiator back up I noticed the hissing again. What should I check next.
I should mention 2 things.
1. I have no radiator shield covering the side where my radiator cap is.
2. When I bought this truck, there was a leak out of the back freeze plug. A sealant was used to stop the leak.

 

Heater controls are all pretty much vacuum powered. (except for temp, which is a cable.) What you are hearing when you turn the truck off is the vacuum bleeding down. It shouldn't really do that.... but, it isn't the end of the world.

 

Bad hose, check valve.

 

I'm a newbie to trucks and mechanics. How do I check my valves?

 

Find check valve in the vacuum line on passenger side of throttle body. Remove it and see if air will only flow one way. If so, it is good. If not, replace.

 

Mine does this, but only on key on, engine off, when I play with the vent dial.

If your vents are still working fine, I wouldn't worry about it until it breaks. lol

 

Read this thread, IIRC there are pics of the check valves and info on how to check them.

https://dodgeforum.com/forum/2nd-gen...ild-vents.html