Was driving down the M4 today in the torrential rain at a steady 70ish mph for about 10 miles. As the trafic slowed in front of me i took my foot totaly off the throtle pedel. This obviously unloaded all the stress on the rear diff. I got a loud horribble grinding/whirring noise from the rear of the car . It only happened for a second and never happened again . I am thinking it was either the brake pads or the rear diff? has anybody experienced similar or could shed some light on what it might be!!