If it's from the rear, it could be shock mounts (deteriorated rubber bushings) or perhaps the exhaust pipe being too close to the shock(s) as the suspension moves up and down. If, perhaps, one of the shocks has leaked out an amount of fluid, that might cause a rattle too. Might also be something like the rear door latch strikers needinig some tweaking for a tighter fit of the door(s)? Depending on the exhaust system hangers, it could be one of them that has deteriorated/failed, letting the pipe(s) drop down too far.
While you're under there, with the car safely parked and supported with sturdy jack stands, you might check the exhaust system for movement by lightly hitting it with the heel of your hand or taping on the shock absorber body with your finger or a mallet--trying to duplicate what you're hearing.
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