it's like a strange high pitched resonating noise. i've noticed it very slightly over the past couple of weeks, but today it got quite loud.
it sounds like the old 'test card' tone on BBC2!
all i can think of is fuel pump going?
I have the been going out of my mind due to the same sound
Asked a Alfa specialist about it, when I was at the garage with a few other annoying sounds: and the first thing the said was the fuel pump!
I had mine replaced a few months back. It gave in after I ran the car very close to dry. That killed it
Not sure if the is a connection though.
Did run the car near dry again recently and the new fuel pump started to make the noise immediately after!
I've heard theories from several sources about "dirt" floating in the fuel tank, getting in the system/pump when trying to run on the last drops and having experienced it twice now I'm beginning to believe
Well try putting your ear to the chassis near the fuel tank cover, when the engine is running.
If you can hear the sound deep in there it's probably the fuel pump
Otherwise the engine going in the stereo system is a possibility as well. I´m in the process of getting rid of that sound as well
Best of luck.