It could be driveline backlash. Is it any worse if you go backwards and then forwards again? If you've got somewhere flat you can try it out. If you're not sure of the service history or even for your own piece of mind it wouldn't hurt to change the driveline oils. The colour and texture of the old oil that's in there now will give you a bit of an idea of what's going on.
Hi,
2013 D40 ST Manual, mine started to do this on fast starts and also with a slight vibration which started at 110kph, found it to be a stuffed driveshaft universal. Replaced it and no problems ever since.
