Cozza

Hi any ideas at all......

Hi Guys. Small problem Jaguar XF diesel when sitting in reverse with foot on the brake just idling I can hear an intermittent knock knock sound from under the car and I can feel it through my feet. It only appears to be in reverse it sounds like a solenoid going on and off, the car dives absolutely fine and when on the move I hear no noises. Recently had a service but cant see how that would have had an effect. Any ideas?