it sounds low down and seems only to occur if one rear wheel hits a bump first. Doesn't sound serious but is irritating. I wondered about slackening off the bolts holding the rear springs and then re-tightening.
Could your rear axle be moving/sliding sideways in the U Bolts? Maybe needs a Pair of Mulfab anti tramp bars fitting, or just new U Bolts. Worth Checking the distances at each wheel arch.
