The carbonating tube leaks CO2 when machine is not in use.

"@jr77 Have you checked out the guide we have here on iFixit. You may find it helpful.

SodaStream Source Internal Carbonation Air Hose Replacement

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 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.

Here is a Ifixit Troubleshooting page you can look at.

Update (01/03/2018)

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.

customer.care-uk@sodastream.com

0845-601-0093"

It sounds like I have the exact same problem. Our machine has only been used for 2-3 weeks. It worked great until it started leaking. I wonder if this a common problem and is it fixable?

No one seem to be answering the question: CO2 leaking from the bottom hole on the immersion tube. This doesn’t appear to have anything to do with the bottle seal.