Hissing Under Dash - Air Con?

[meb90]

Hello all,

 

Looking for some advice. My Mondeo mk4 has recently started making a hissing (best way to describe it) noise coming from under the dashboard. It stops when I turn the air con off, and returns when I turn it on.

 

I bought the car in August last year, and had the air con re-gassed, it is likely the issue is with the evaporator? Its difficult to tell, but I'm sure the air con still works and I haven't smelt air con gas inside the car.

 

Your thoughts are gratefully received.

 

Thanks,
Matt