My Dart has this also has a bit of in and out 'phasing' rumble when cruising at the same speed at lower speeds.

But I know the front u-joint has some little play and this started occuring when I reversed the way u-joint was installed recently. It also had some visible wear on the crossjoint itself. I will replace it with a new one.
I also know a header tube is very close to a torsion bar and it will sometimes vibrate slightly against it, but that's not the cause of the driveline-'phasing' I'm feeling at times.