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Mystery Noise Under Hood


SCJax2018
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This morning I drove my 96 Pathfinder LE 3.3L about sixty or so miles. I stopped and shut off the engine and about two seconds later this barely noticeable noise came from under the hood and it sounded like a fog horn in an old movie or cartoon, but it only lasted a few seconds like it started and quit before you could say "What the ......".

 

I am totally puzzled because I've never heard a noise like this come from under the hood, so I have no clue if it is a major issue to be concerned about or just a minor issue. The only thing I can think of that makes any sense is that it is some kind of vacuum leak on one of the vacuum canisters. Anyone got any idea what could be the cause?

Edited by mjotrainbrain
Tidying up after merge
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I don't know if it's the same noise, but my '93 makes a sort of groaning duck noise from the fuel tank area sometimes after I shut it off, or even sometimes when it's just sitting there on a hot day. There's a check valve in the evap line to the charcoal can that prevents the tank from draining fuel into the canister if you roll the truck, and I guess it takes a certain pressure to lift the ball inside and let fuel vapor through--so pressure builds until the valve opens, and then the valve kinda flutters and makes the groaning noise. I actually replaced the valve on my '95, thinking the noise meant there was something wrong with it, and the replacement made the same noise. :doh:

 

I don't know the R50 evap system but wouldn't be surprised if Nissan used a similar valve.

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1 minute ago, Slartibartfast said:

I don't know if it's the same noise, but my '93 makes a sort of groaning duck noise from the fuel tank area sometimes after I shut it off, or even sometimes when it's just sitting there on a hot day. There's a check valve in the evap line to the charcoal can that prevents the tank from draining fuel into the canister if you roll the truck, and I guess it takes a certain pressure to lift the ball inside and let fuel vapor through--so pressure builds until the valve opens, and then the valve kinda flutters and makes the groaning noise. I actually replaced the valve on my '95, thinking the noise meant there was something wrong with it, and the replacement made the same noise. :doh:

 

 

 

I did notice when I fueled up the other day that the tank had some pressure when I unscrewed the gas cap.

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That may be the cause of my noise. Sorry if this font is a little large, I don't have my glasses with me and had to blow the font up a bit so I could see what I am writing here.

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I bet what Slart said is more likely than what I’m about to say but from time to time when I shut off my engine after running the AC the low and high side will equalize and sometimes it makes a sound just like those little paper party blow horns that roll out like you’d see at a kids birthday party.

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2 minutes ago, onespiritbrain said:

I bet what Slart said is more likely than what I’m about to say but from time to time when I shut off my engine after running the AC the low and high side will equalize and sometimes it makes a sound just like those little paper party blow horns that roll out like you’d see at a kids birthday party.

 

This sounded like a fog horn from an old movie or cartoon. It wasn't loud loud, just barely audible, but loud enough to make you say "What the ..... ".

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Yep, that sounds about like mine! Being concerned about it didn't change anything the last time and it doesn't seem to be hurting anything, so I'm not worried about it. It doesn't happen very often, either. Sometimes on a hot day I can shut a door and the vibration starts the valve making noise, which is amusing sometimes. Like, man, what am I feeding this thing?

 

The only common fault I've read about with the R50 evap system is the carbon can barfing its guts into the purge valve, locking it up and throwing a trouble code. I don't think it's got anything to do with the noise, but here's a writeup on fixing that issue if you're curious about the system.

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18 hours ago, Slartibartfast said:

I don't know if it's the same noise, but my '93 makes a sort of groaning duck noise from the fuel tank area sometimes after I shut it off, or even sometimes when it's just sitting there on a hot day.

Ive had this happen to me on my 01. I have video of it that I'll dig up and post soon. Nothing to worry about. It did throw me off though as this is the only vehicle I've ever driven to do this. 

Edited by TowndawgR50
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29 minutes ago, TowndawgR50 said:

Ive had this happen to me on my 01. I have video of it that I'll dig up and post soon. Nothing to worry about. It did throw me off though as this is the only vehicle I've ever driven to do this. 

I never had one do this before either.

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