Are you sure its an airbag hissing and not the connections at the compressor, given you have just changed it?

Rear airbags can also develop leaks. Start car, put in off road mode. Open door, switch off car. Have a good listen around the rear end. If you can have a feel around airbags at back for air coming out (More difficult on rear ones than the front ones). Also try lifting at wheel arch to see if hissing sound gets louder. If you have a hole, and you only get it in off road mode, this is because the airbag rolls itself over the hole when in road height mode.

Also leave in off road mode overnight, see if you go lop-sided then. You may go down on both sides at the back due to the way the cross link valve works.

Rear bag exchange is an easier (and slightly cheaper) job than front ones because the shock absorber is separate. .
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Thanks, yes it's definitely a leak not the compressor as I have this a while now but getting worse.

I'll leave it in off road overnight see if it goes lob sided. I can clearly hear it when I get underneath its v loud. I just can't pinpoint it. I had a look at eBay and they seem to be relatively cheap for seconds.