Noticed recently, that after turning the engine off, there is a hissing noise from somewhere under the bonnet that lasts for about 15 seconds, then dies away over 3-4 seconds. Sometimes think I can hear it randomly inside the car when driving, again last for maybe 20 seconds then fades. No other apparent symptoms, no liquids appearing anywhere - any ideas??? By the way, which "w" variant engine am I likely to have? Thanks.