Passat loves Wob Wob Wobbing when stationary!?

[Ollie00]

I have a 2.0 Passat 51 plate coming up to the 100k mark
Over the last few days my passat has started to make a wob wob wobbing noise when stationary, never when starting up but after i have been driving for some time.
This only happens on the odd occasion, either its fine or this wob wob wobbing noise appears (like some kinda spaceship).

Anyone any ideas?

Thanks in advance,
Ollie