ok do this,
open your boot
pull the carpet wall stuff away after taking the little plastic rivets out
stick your arm under and down into the panel below the jack is.. and see whats down there
i had a rattling in mine only over certain bumps, it really started to piss me off.
i stuck my arm into the panel, searched around a bit, and found 2 bolts, a spanner and a ball point pen
problem solved.