rp911

My AC is working fine, and blowing cold, but just started making a momentary hissing sound audible in the cabin every so often. Anyone encountered that before?

Ed Burdell

Mine does that. Almost like a chirp.

ToreB

That's probably one of the air valves working, opening and closing of either hot air from the engine, (one of the mixing valves) or the fresh air servo. This is normal behaviour, as to regulate the temperature in the cabin.The hissing is probably caused by a dirty or damaged air valve seal, often a thin foam stripe around the edge of the valve flap.
You can temporarily stop the periodic hissing sound by setting the temperature either higher or lower.

It's easy to spot which servo/valve that is causing the sound, have a look in the trunk or footwells while the engine is running. But, please note that if you do this inside your garage you will probably have different surrounding temperature and the movement can stop due to the different regulation needs. You may have to adjust the temp setting to replicate the motion with different outside temperature.

I have had success in cleaning the valve seal in question to get rid of the hiss, that was the fresh air flap. Have a look here for details in servo/valve positions:
www.ccu.tore.bergvill.com
Cheers,
Tore

rp911

Thank you for the link, Tore. It was very informative. However, I'm not sure it has anything to do with temperature regulation servos, as it was on an extremely hot day with the system set as cold as it goes, fan on the third speed, and the recirculation button pressed. I would think under those circumstances the valves would remain in the same position. As I said, the AC is cooling well, and turning off the AC button, the hissing stops. The hissing is for a second or two, and recurs every couple of minutes.

ToreB

The resirc flap is vacuum operated, maybe you have a vacuum leak somewhere? This would make the flap saggy and it may be slightly moving due to other vacuum operated mechanisms in the engine or variations in vacuum due to acceleration.
A malfunctioning resirc flap will force the interior fans to suck air from closed air vents, this may cause a hissing sound.
Cheers
Tore

ble2011

Mine does exactly the same thing. All control settings are just as you described. It's audible in the cabin, but more noticeable standing outside (listening over the windshield). About a 2 minute interval - hissing and then fading away. I think the previous posts were dead on, probably a vacuum leak somewhere around the servos. Not going to be able to check it right away, but I will update what I find.

Stewart

I'm glad someone else has the same issue. Mine has got exactly the same symptoms. I only recently noticed it as daytime temps have got that much higher - I guess I'm using the aircon more often now.

rp911

From reading online (mostly about other cars), it seems that it could have something to do with the expansion valve and the sound the refrigerant makes as it changes from a liquid to a gas. Apparently, if the charge has gotten a little low (and mine has not been topped off in years), the liquid refrigerant can get a little frothy and cause that hissing noise. i guess the first step would be to have the charge checked out.