Rear hissing

[D3v7n]

Hello Hilux friends

I have a problem I hope I can get an answer to,
After filling up I went onto the freeway and after about 10min of driving at 110kph I heard a hissing sound getting louder only when accelerating up hill and went away downhill. I pulled over and the hissing was coming from near the rear right wheel, it remained for a couple minutes after the engine was off then went away. I also noticed a couple drips of what looked like brake fluid but was dripping from the chassis near the shock, it was literally 2 drips.
When I got home I noticed it only hissed on the freeway and only up hill, not driving around town under 80kph, at home with the engine off the hiss remained for a couple minutes then went away but no signs of any drips this time.
It’s not a on/off hiss, it builds up and down. Any ideas?
2008 2x4 Petrol Manuel Hilux

[dave g]

Try opening the filler cap and see if it stops
If it does you have a block breather on the fuel tank.

[D3v7n]

Thanks Dave, I’ll give it a crack

[D3v7n]

Hello, here’s an update,

Went for a drive on the freeway, about 5min in the hissing sound started up. I pulled over and opened the fuel cap which did nothing to the sound so I guess it’s not the fuel tank.
I also noticed that the sound is louder on the drivers side rear. On my way back I put it in neutral down hill and the sound pretty much went away but then slowly came back after accelerating up hill.

What else could it be?

[DeadlyBeast]

? Diff breather blocked
Might pay to check your diff fluid level too

[D3v7n]

Thanks for the tip DeadlyBeast,
I changed the diff oil, cleaned the breather, both were fine.
Then took her for a test drive and same thing same place, 5 minutes in, going up a hill the hissing builds up.
Definitely louder on the drivers side rear, sounds like it’s in front on the rear wheel.
Someone suggested a partially blocked muffler? But would it build up pressure like that?

Thanks for all the help so far!

[DeadlyBeast]

What modifications are on the vehicle? How many km? Gone thru and mud before the sound started?
May be another breather around the transfer case area
The good news is that it is making a noise so you can identify where it is
Too hard to guess without being there or more information

You just need to get under there and find exactly where the sound is coming from

[D3v7n]

It’s the 4.0L RWD model so no transfer case, haven’t driven through any mud, first time on the freeway in about a month, just been driving around town (short trips).
Done around 145,000km and the only mod it has is 19inch rims.

[D3v7n]

Update:
Took her to an exhaust shop to check the muffler, they got their mechanic to do a test drive.
Came back and said he couldn’t hear any hissing, so I took her out on the freeway thinking he didn’t do it right and I couldn’t get the hiss either. So she’s fixed her self?
The mechanic was stumped as well.

I’m going on a short trip on the weekend so we’ll see if the sound comes back.
Thanks for advice guys!

[DeadlyBeast]

Totally missed it was a 2WD.
Its a strange one. Cant think of any thing else that would cause hissing for minutes after stopping engine. Its gotta be related to heat build up , and air movement in and or out. Get prepared to roll under there with a wet finger and keen ear to pin point it if it happens again during the weekend trip. Good luck