This may be a stupid question, but here goes. The A/C in my car has been making a hissing noise for about the past two weeks. When I use only the vent, the noise isn't there; only when I actually turn on the A/C. It's not cooling, either. Obviously something isn't right with it, and I know I need to get it looked at.

I think I might have an idea of where it came from. At the very end of February, I took my car in to get inspected, and they needed to replace the A/C belt as it was cracked (I know, I know, I probably could've had it done cheaper elsewhere, but I was in a hurry and didn't have time for that, and I also trust this place a fair amount). I hadn't noticed the hissing noise before that, but then again, I hadn't really used the A/C too much lately due to the cool weather. But I don't recall hearing the hissing noise until after the inspection.

So...is there a chance something went wrong when they replaced the belt? If so, I'll take it back and have them look at it. If not, then I'll have my mechanic (who I trust completely) look at it this weekend.

Any thoughts, Auto board?

Edit: Sorry. It's an '03 Subaru Legacy, with about 43,000 miles.

[This message has been edited by coppertop (edited 3/11/2010 9:16a).]

[This message has been edited by coppertop (edited 3/11/2010 9:47a).]

The year make and model of your car would really help us help you to diagnose.

When do you hear the "hissing" sound?

If it is AFTER you turn the car off, that is perfectly fine. That is the pressurized gas depressurizing in the expansion tube/orifice. You can't maintain the pressure difference without a valve in the way.

You would normally be able to hear it, but with an engine turning, it makes it a bit difficult.

If it is inside the car when you are driving, you might be hearing a vacuum leak (really big one, unless your interior is pin drop quiet).

I would need more info to figure it out.

~egon

It's only when the car is on. And even then, it's only when I turn the A/C on. If I just use the vent, the noise isn't there. But once I hit the button to turn the A/C on, it starts up; hit the button to turn the A/C off, the noise goes away. The noise is also there when I turn on the defroster, but it goes away when I turn off the defroster.

I would say that you are just hearing the freon expanding (and cooling). There could be a vacuum leak (turns on with the AC system), but I wouldn't think you would hear it.

Is it really loud or fairly quiet (radio off, not moving, etc)?

~egon

It's not terribly loud, but it's noticeable. I can hear it when the radio is on, but I don't turn the volume up too high, and I mostly listen to talk radio. When the radio's not on, it's definitely noticeable. The A/C isn't cooling at all, though. It was a bit chilly when I drove to work this morning, so I put the heat on for a few minutes. The noise was still there, but it seemed a bit softer, and the heat function seemed to work better than the cooling (that is, I actually felt a bit of heat, whereas I don't feel much cold air at all with the A/C on).

To fully tell you what is going on, I would have to put gauges on the system and look at it. You could be a bit low on freon, hence the lack of cooling. Was the heater on or was it the defroster? Defroster usually turns on the compressor whereas the heater won't.

~egon

Where is the noise coming from, inside the car or outside? If outside, the belt is probably slipping. If inside, remove the glove compartment and see if a paper has fallen out of your glovebox and blocking the intake to your AC evaporator.

I had this EXACT problem with my 09 jeep wrangler unlimited. It would hiss only when I would turn the AC on and the sound would come right out of the console. It wouldnt happen if I just used the vents.

I took it to the service dept and they said it was low on freon. It hasnt made the noise since.