Skoda Octavia noise in transmission

A curious noise has appeared, apparently front O/S. It is not present on over-run, but is heard (even felt, slightly, through throttle pedal) under 'neutral throttle'. It is still present, though somewhat reduced, under acceleration. The noise sounds (a little) like a CV joint on the way out - but the CV joint has just been replaced, and the noise was there prior to that.

The noise may be described as a 'cyclic rough running' noise, ie it sounds as if something is running rough each revolution of the drive shaft under certain conditions. It is absent if in neutral whilst the car is in motion, or, as above, on over-run.

Steering alignment has been set up (laser system).

The car has now covered about 500 miles since the (intermittent) noise above was first noticed and has got no worse.

It may or may not be relevant to mention that for some time it occasionally happens that the car has suddenly feels as if the 'back end' has stepped out for a moment - a distinct queasy 'twitch' occurs. All linkages, bushes, etc etc etc have been checked - nothing amiss noted.

My best guesses are - intermittent engagement of Haldex unit (no indication on warning lights on dash however) - or, fuel sloshing about in tank - collapsed baffles (?).

All / any feedback most welcome! Thank you.