The buzzing sound you may be hearing is the vacuum line to the turbo actuator. After engine is turned off it takes a little while for the vacuum to equalise. To test this theory, when you shut down engine and you hear this noise, open bonnet, providing you are facing the front of the vehicle, to your left is your turbo, keep looking left and right along the drivers side quarter panel, about in the middle, just behind air box, there is a little black hose going in top of a little black cylinder. If you pull this hose off the top of cylinder and the buzzing stops, don’t worry, this is what the noise is and what it should be doing.
Remember to connect this hose again, or you will have an engine light on and no boost for your turbo.
Hope this helps. GL