To the sound guessers!:)

hello!
'I've been hearing a noise in my Golf with the 1.6 TDI engine for quite some time now, and I can't identify it: when driving at a constant speed, there's a faint, sewing-machine-like hissing sound that mixes with the background noise. It disappears if you barely press the accelerator. When you give it more power, it reappears and is always the same volume, regardless of whether you apply little or a lot of gas.'
It's definitely not the sound of an exhaust leak; it's more like a muffled, shallow rattling. Do fuel injection pumps make noises like that when they are faulty?

It could simply be the needle bearing of the gearbox shaft... there are many possible causes, but a remote diagnosis is difficult.