Put your van's transmission in neutral / free, and then try to move the propshaft in both directions. With both wheels on the ground, there shouldn't be much play, and there should not be much sound.
If you notice something it could be in the shaft itself (splines), in the joints as mentioned, but also inside the differential.
About the repeated throttleing and releasing, if this also causes the effect/sound when your stationary, with transmission in neutral/free (or clutch pressed), then you should check all enginemounts and gearbox mounts.
If it is none of the above, and you cannot reproduce the sound while being stationary, then you should check suspension.