Why do I hear a hissing noise coming from my SodaStream?

"Sodastream has a pressure relief valve, so it is important to see where the sound is coming from. It’s normal for gas to come out of this pressure relief valve. Make sure that your carbonation cylinder and carbonation hose are not leaking. You can easily check it by taking the covers off.

If you are hearing a hissing noise coming from your SodaStream Source, the carbonation tube is leaking gas, or the carbonation bottle is leaking, use the SodaStream Source is Leaking Gas problem page to fix the leak.

If you are heaving gas leaking from the carbonation tube, use this SodaStream Source Internal Carbonation Air Hose Replacement Guide to replace the device’s internal carbonation air hose. This solution requires taking apart the SodaStream Source."