If the Carbonation canister is not set firmly, you may hear excess gas leaving the machine. This can be caused by having worn out or too many washers, or if the canister is not screwed in enough. Make sure the canister is securely fastened before and after use.
*Note: Recent carbonation bottles have been recalled due to the possibility of bursting under pressure. If this issue of gas escaping occurs, and your bottle was purchased between Feb. 2016 and Jan. 2017, consider changing out the canister and checking if your bottle was part of the recall.
[[SodaStream Source Troubleshooting#Section_Carbonation_Canister_is_Loose|Here]] is a Ifixit Troubleshooting page you can look at.
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=== Update (01/03/2018) ===
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If changing the cylinder does not help then it is most likely defective. It is most likely a leaking gasket or o-ring. It may be repairable but It is dangerous to mess with anything that uses that kind of pressure. I would replace it myself if I were you, but before you do try the soda stream support team and see if you can get a replacement.
If the Carbonation canister is not set firmly, you may hear excess gas leaving the machine. This can be caused by having worn out or too many washers, or if the canister is not screwed in enough. Make sure the canister is securely fastened before and after use.
*Note: Recent carbonation bottles have been recalled due to the possibility of bursting under pressure. If this issue of gas escaping occurs, and your bottle was purchased between Feb. 2016 and Jan. 2017, consider changing out the canister and checking if your bottle was part of the recall.
[[SodaStream Source Troubleshooting#Section_Carbonation_Canister_is_Loose|Here]] is a Ifixit Troubleshooting page you can look at.