240D engine making buzzing/farty sound

[efm-7]

Something I noticed today while checking my injector return lines (which are wet),

I'm getting a buzzing, almost a high pitch farty sound from the engine, or somewhere around it.

it's intermittent during idle and constant when I bring the RPMs up.

Any idea what this could be?

I took a video, please have a look

http://youtu.be/pppl2MascDE