There are some bushes at the rear end which if they fail, they pull out of the arms. Originally there are large washers which would prevent the arm from coming off the bush but if the back end has been apart before and put back together with the washers in the wrong place it'd be possible for a suspension arm to be flapping about.. I would get underneath and have a look before driving it somewhere, could be dangerous. I've seen it happen on a car we bought for breaking and one of the rear wheels simply folder underneath the car while cornering, not pretty.
Its probably not this, but better safe than sorry.
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