I've noticed lately, but been very lazy not to find and fix this problem, but today I had my exhaust manifold downpipe gasket sorted (see my other post), and thought i'd remove my plenum and check the air pipes are all ok as I have a rather nice but alarming sounding 'pshhhh' sound when I got the car upto over 4k revs and let go of the accelerator - I know this is an air leak, but for the life of me and believe me I checked every air line and vac pipe under the bonnet, I couldnt find the fault. I replaced the O rings on the plenum and no air sounds to be escaping or sucking from that area, the pshhhh noise is only evident when the car has been under a heavy load and gas pedal released so I cant get it to make the noise while idling no matter how hard or high I rev it.
I havent got a turbo under there before anyone suggests

and the air intake system - well the whole engine is standard so nothing has been chavved and it will stay that way as long as I have it

Come on lads, point me in the right direction :-/