Double bonk/tapping noise when I first pull away

When I initially pull away, after abt 50 metres, I often hear 2 tapping/bonking sounds - seems to be behind the dashboard at bottom right. Sometimes, the sounds seem to come from behind me. Garage has checked- nothing is loose. They are clueless. They did affix airco pipe, but problem persists. They are referring it to VW. Their loan car does it too! Anyone heard of it? Any ideas? It never happens during a journey, just on initial pulling out and accelerating away.