Will check again but think its blowing as opposed to sucking at idle... not sure... thanks for the replies and ideas - will check it out and post results.
Couldn't see any other leaks Tab - and she ran beautifully even after the hiss came in at a junction for the first time. As smooth as can be apart from that, even revved beautifully under load etc.
I haven't changed it recently Rinus mate - have been running on it for around 800 miles with no probs and it didn't hiss before yesterdays blast - but what a blast it was - luverly
Get back to you all - thanks again,
Andy