I work at a dealership and my 08' Land Rover LR2 @ 87,xxx miles just started doing the whistle while idling this this past weekend.
Had a one of the mechanics take a look at it and from my understanding the PCV Valve is whats causing this whistling sound. Apparently over time the engine has a build up of oil and the part connecting it to the dipstick is making that sound because it needs to be replaced. Everytime he lifted the yellow dipstick thing the noise stopped. That said the PCV Valve is built in with the Cylinder Head Cover and needs to be replaced all together. See link below for what it looks like and what's needed:
#LR023777: Land Rover. Engine Valve Cover. VALVE COVER.
** This was the best link I could find that looks like what my mechanic pointed out/printed out for me from the parts guys.
** When looking at the link scroll down to #4
& 5 this is what my mechanic told me it is. Either way I would have a mechanic check to make sure, but the part cost me 136.00 from my part guys