Sent my car for my 1K service, and I asked them about this. I too, always hear the 'TUNG' sound when my speedometer reaches 40KM/h. Apparently, that's the sound of the VSA relay kicking in, and according to them, it's perfectly normal.

Sounds plausible?

Immediately after freeing your vehicle, be sure to switch VSA on again. We do not recommend driving your vehicle with the VSA system switched off.

You may hear a motor sound coming from the engine compartment while system checks are being performed immediately after starting the engine or while driving. This is normal