Strange Noise After Shut Down

[SCJax2018]

When I stop my 96 LE, every once in a while, usually after a long ride, I hear this strange noise from under the hood sort of like the low tone of a fog horn. It isn't super loud and only lasts a few seconds like pop the hood latch and by the time I get the hood opened it has stopped. At first I thought it was the radiator cap, so I replaced it and I still get the noise from time to time. What could it be? Leaking vacuum line line somewhere or a vacuum door on something?

[mjotrainbrain]

I remember seeing this discussed before but I don't remember what it was; I do remember it was a non-issue though, just a normal thing (EVAP depressurizing or something like that?).  Maybe I'm wrong, but that's what I seem to remember.

[Slartibartfast]

Mine makes a noise like that from the back when it's hot outside. Tracked it down to the check valve for the fuel tank (prevents fuel draining out through the evap system if the truck rolls over). Replaced it on my '95 and the new one did it too, so I haven't bothered doing anything about it on my '93. I'm not sure where the check valve is for the R50.

[SCJax2018]

21 hours ago, mjotrainbrain said:

I remember seeing this discussed before but I don't remember what it was; I do remember it was a non-issue though, just a normal thing (EVAP depressurizing or something like that?).  Maybe I'm wrong, but that's what I seem to remember.

 

I thought I have posted this question before, but I couldn't find the post.  It's been doing this for quite a while now, but it seem to be louder in warmer weather.

 

I haven't checked the check valve. My tank does have a lot of pressure at times when I screw off the cap, but I've been told by several that is normal because it is a pressurized fuel system.