Aircon hissy fit.

[Circlotron]

When the a/c is running and I switch the engine off, you can hear a gradually decreasing hissing noise behind the dashboard somewhere. I expect it is inside the a/c where the liquid squirts through that orifice that turns the high pressure liquid into a low pressure gas and gets cold.

Also, when you give it a bit of a rev up when you move away from the lights somewhat energetically, you can hear a slight squeal from behind the dash as presumably the gas pressure builds up. It works perfectly alright as far as I can see.

Are all VN/VP aircons like this or just mine?

[stockies]

Mine has the first issue, but not the second. I can hear what sounds to be running water behind the dash all a lot. Happens more when the dial is set to hot though...'
I think its a problem with the heater tap and/or heater core...

[Circlotron]

Okay. Thinking more about it the squeal noise is actually a whistling sound. Yeah.
I don't think mine is a heater tap or core problem because when I use the heater alone it is all quiet.

[rambunctious]

Circlotron, the noise you hear is most likely vacuum being sucked through a hose which has come away from the air con control valve in the dash, and this is also the reason the noise changes when you rev the engine and change the vacuum flow.

You may find a vacuum hose is split where it attaches to the control switch.

You need to remove the control switch and reconnect the vacuum hoses in their correct place or cut off any split ends then reconnect.

[Jxw]

first problem, yeah that's normal i've heard it a few times myself.

2nd as above look for a vacuum leak, do all the vent controls work properly? also the selector can brake a clip that holds it all together and leak in certain positions.

[Circlotron]

Hey, that is interesting guys! Yeah, all the vent controls do seem to work okay. What I'll do next is rotate the selector to a/c and turn the car off and let it hiss. Then come back later and without starting the engine, rotate the selector and see if I can hear anything operating. Usually with face level vents selected for example and a/c off I can come back next morning and there is still enough vacuum in the system to select this or that; at least it sounds like that because you can hear a little "chiff" as the selector is rotated.

[Jxw]

that's a good test, if it's like that then it won't be a vac leak, i've noticed a high pitch noise in my 88 vn when cruising at certain speeds and it changes pitch upon accel or deccel, never paid much attention to it usually turn the stereo up louder as everything works correctly and last time i was behind there looking i couldn't see any obvious signs of a leak.

[Circlotron]

What I'll do next is rotate the selector to a/c and turn the car off and let it hiss. Then come back later and without starting the engine, rotate the selector and see if I can hear anything operating.

I let it sit overnight and there was still heaps of vacuum in it so there are no leaks. Today I popped the bonnet and with the a/c running, switched off the engine and jumped out and had a listen. Sure enough, you can hear a hissing coming from near the receiver/dryer thingy. End of story.

[Sharpie]

Normal its the a/c system equalizing in pressure. Its the way the system was designed.