My 2011 MK4 1.6 td I had an intermittent high pitched whine when bonnet is open and engine off. Key out.

Sounds like from area of camshaft at timing belt end of head. Maybe cam position sensor ? 
 

Very high pitched, fairly quiet and not continuous.

Any ideas ? 
 

I know as well there is a cylinder head temperature sensor for the ECU )as distinct from the water temperature sensor con the plastic housing at the clutch end of the engine), but unsure where that sensor is.

1.6 Econetics seem thin on the ground.

Certainly pulls well for a small turbo diesel in a family sized car.

 

Cam sensors don't generally whine.  It's usually solenoids that whine.  I suspect it's the fuel pressure regulator in your case.  That's on the high pressure fuel pump in that area.

It’s very odd. Not loud. High pitched. On for a minute or two then off but comes back on.

Recent fuel injector replacement but was doing it if bonnet was open before the injector was swapped. 
 

I wondered if it could be from AC. My directional hearing is not great 

And this is when key is out and same if engine  has been off for ages. Like a day or two. No noticeable effect on battery. 

The sound deadening cover was off the engine. Might well not be loud enough to hear when it is on. 

Will have a look for HP fuel pump. 
Thanks. 

Puzzled as to what could produce a noise after several hours when engine off key out and bonnet open. I guess there’s a bonnet open switch for the alarm but the alarm isn’t going off.