[5potnoodle]

Had this problem on my 200 once... the intercooler to intake rubber flexi bellows (michelin-man) pipe thingy had split but it wasn't noticable. I got the hissing and sounded like a dump valve when changing gear. I think somebody mentioned fingers in dykes... Get your fingers around the pipe on the bottom side and feel for the split. Mine had split underneath one of the metal rings on the underside of the pipe, couldn't see it but finger went in. Took it off and temporary repaired with a rubber o ring and some bycycle tube glue which worked for a while. Audi quoted me some ridiculous price but sourced an OEM replacement from the US, brand new for about £30.