The other post about buzzing reminded me of something I've been puzzling about for a while. My old 2003 Fiesta Zetec makes a strange little faint buzzing/beeping noise from under the bonnet for a few seconds when you switch off. You can only hear it when you get out. I'm not sure if it's every time, or just sometimes - it's so quiet that it's easily masked.
My searching found some results mentioning 'rail volume discharge', and I'd say that my description matches theirs, but in each case, they were all diesels. Would my petrol model do it too, or is it likely to be something else?