it's the weirdest thing but i've found the culprit.
there's a little roller switch on my control consol with a light symbol on it. i'm not sure what this is for, i am guessing dashboard illumination, but since i currently have no illumination in my dashboard i can't tell.
anyway, for some strange reason, if this switch is not rolled into the top position the car makes a strange hissing noise!
if anyone can tell me what this is all about i would be very interested to know. at least in the meantime i have a bit more peace and quiet.