Sharan 4motion TDI with rumble sound in the rear

I've got a Sharan 2002 TDI 115 4motion that makes a strange sound as follows :-
when you move off at an angle, (also in reverse), there is a loud rumbling/booming sound coming from the rear of the car. It doesn't happen at speed, only when moving off from stationary.

When driving at 130km/h and the way goes a little climbing, and you are in 5th gear, there is also a great "bass-sound " in the rear.

My local garage hasn't got a clue.
Has there anyone a solution for this problem

Cheers,

dekleinebach

I had the same on my 4motion V6, was at it's worst when reversing on lock.
It happened after fording a flooded road, so assumed water got sucked into the Haldex breather.
I had the Haldex oil and filter changed and it went away and hasn't returned.