high pitched noise from rear

i have been to my audi dealer with this noise problem they replaces a modified fuel hose but the noise is still there. the noise comes from inside the passenger wheel arch, its sounds more electrical that a vibration and when i rev the car the noise gets louder and higher in pitch. it is an intermitent noise some days its worse than others, but its very irritatating, one audi mobile technician said it could be a faulty fuel pump but at the main dealer they said there is no fuel pump at the rear of vehicle. so does anyone have any ideas not just for me but for the audi dealer??? by the way my audi is a fantastic car apart from tedious faults like brake light sensor failing, loose fuel pump creating noise vibration under bonnet, and the high pitched noise!!!