I've noticed a weird whistle noise coming from my engine, it only happens as I accelerate, it's more apparent under gentle acceleration.
It's sort of mid pitched and warbles at the same time.
air leaking out the turbo!!!!!!
There's a duck caught under you bonnet silly.
Does it do it with your radio off? If not then it could be interference from the alternator.
Warbling sounds can also be a sign of a worn /nearly dead turbo or one thats lost one of its fan blades.
I'd go to my trusted local independent and ask them to have a listen.
I had a Galaxy Tdi that threw its turbo fan into the engine inlet, expensive noise followed