Hi I'm new to the forum I own a 54 plate superb 1.9 TDI after a drive when I switch engine off I can hear a hissing noise I can hear it thru the vents and outside the car. Is it just air con decompressing or is it something to worry about

Probably the A/C.

 

Try turning the A/C off five minutes before you switch the engine off to see if that stops it.

Mine does the same think I've been told it's nothing to worry about.

Mine does the same think I've been told it's nothing to worry about.

 

Its the vaccum system fed (sucked) from brake servo "T" off into pipes into a pipe system that operates EGR, Variable Vanes and anti shudder valve using solenoids with a vacuum resevoir

 

It can spring a leak in the flexible or fixed pipes - one of my fixed pipes rubbed through because of vibration - easy fix - showed up as EGR fault(it was fine) and engine shuddered on turn off

 

To ensure all well make sure anti shudder valve is operating - this will show vacuum operating. With the bonnet lifted looking from front towards windscreen this is the vlave thats visible to the left of the engine cover. The plunger should operate as you turn the engine off - oh and of course the engine should stop without a shudder

My engine doesn't shudder