I'd like to hear that....there is not much there ...do you have auto-leveller control? If so, could that be the compressor that "adjusts" vehicle height on first acceleration since that causes the rear end to dip down...and once done, it would not need it for a good length of time...unless it has a leak ...
Other than that, I'd think of a rattle from exhaust heat shield.....so...if you can record the sound and post the sound stream......