Our air con runs, on heat, and after a time cuts out, blows cold and makes a sort of hissing noise (but not a real hiss) and then shuts down. It then restarts after a couple of minutes. Is this normal?

Check the bearing in the end of the blower motor. They go bad and make a noise and over time will start rubbing and squealing like crazy.

Not sure if I'm too late to answer this one, but you're describing a low refrigerant condition. The hissing noise and constant cycling of the compressor clutch are indicators of that. You should recover the system, record how much refrigerant was recovered, evacuate the system (with a vacuum pump), add A/C dye, and recharge the system to capacity. Common leak areas on these vehicles include the A/C condenser (located in front of the radiator), and the evaporator (located inside the dash, and requires complete removal of the interior dash to replace it). If you can't find a visible leak after running the recharged system with dye for several weeks and it starts to blow warm again, you will need to use an R-134a leak detector to "sniff" areas of the system for a refrigerant leak.

Some of the Vehicles with Air Suspension require the Pump to be shut off , before you lift the Wheels, off the Ground. The hissing noise indicates the bags ruptured and need replaced, at the very least. If there is an air suspension Cut-off, it is usually right front Kick Panel Location. If not you made need to pull a Fuse. Without a Manual, you are kind of hosed. Buy a Manual , for around $30, at the Local Parts Store.

Hello Scott,Your new ac is OVERCHARGED with freon.Shut it off,let it cool down and recover/remove freon.The compressor has an internal pressure valve that makes the (hiss).It will stop when the charge is adjusted.Call them back to do it.

i beleive these are a vacuum system. if air temp is in cold position you will get no heat. listen for hissing noise with motor turned off but key on. if you hear hissing noise you have a hose broken or off. you will have to find it

Strongly suspect the AC system has sprung a leak somewhere and the refrigerant is 'low' - the 'hissing' noise you hear is normal when the system is 'undercharged.' Solution is inspect the system & repair the leak. If it turns out there is NO leak, then the problem is either at the compressor (low head pressure) and/or the expansion valve at the evaporator.

Sounds like you got a leak all right Check compressor clutch as well Motor revs should fluctuate when compressor cuts in and out Could be the seals in the compressor as well You need to find where the hissing noise is coming from

hello , sounds like an air lock in coolant system , drain system and re fill very very slowly , if you think you are filling it slowly enough slow it down even more , also replace thermostat to ensure its opening and closing properly , hope this helps

yes that is the expansion valve! there is a hissing sound from the restriction in it for a pressure drop thats what makes the a/c cold the hissing is normal

This may be the refrigerator itself making that hissing noise, which is not uncommon.
Please provide the make and model number for the unit.