I have a 306 diesel turbo. About a week ago I noticed a slight hissing coming that appears to be coming from behind the dashboard. I have noticed that it goes away when I rest my foot on the brake pedal though. After I turn the engine off it remains for a minute or so, then goes. Took a look under the bonnet and though I can hear it I cannot work out where it is coming from. Brakes appear to work fine. Any suggestions would be gratefully received
Cheers
Tony