Subaru Forester 2013: Rattle over bumps

My 2013 Subaru Forester has developed a rattle that seems to come from the left-hand rear of the car when driven over sharp bumps. I have removed all the spare wheel gear, lowered the rear seats, run the car with the rear doors and hatch open, put the car up on ramps to see if it's coming from underneath the car, but I've had no luck in finding the cause. I mentioned this to my dealer and all they did on the first service was put some double-sided tape under the rear number plate. No good. I would appreciate your help.