Just in the last week I have noticed that my truck makes a weird hissing type noise after I shut it down, almost like air being released for a moment. The sound itself only lasts a few seconds, then the truck is silent for a min, them the sound repeats. This happens for 20 or so mins after shutdown, almost until things have cooled off. Sound seems to be coming from underneath around the transmission area. I have no idea what under there could be building air pressure when I drive and then bleeding off. I will attach a vid.

 

Here are some videos. Sounds like letting air out of a balloon

 

Tracked down the balloon deflating noise. Coming from the flex pipe just before the muffler. Have no idea why it does this as it cools down. Also not sure if it just started doing this and why, or if it has always done it and I have somehow not noticed it

 

Really?! that's very weird, i've heard all the cool-down contraction noises over the years, but this one? wow.