This is natural. Water contains gases that are dissolved in it from the air. When water is heated these gases try to escape. They can't, because they're under mains pressure in your combi. When you turn the tap on, the pressure is reduced, allowing the gases to escape as tiny bubbles, giving the water a milky appearance. Normal, harmless, and as you say, the water clears, once the bubbles are gone. It didn't happen on your old system if it was a hot water tank or immersion type, since a vent pipe would have allowed the gases to escape as the water heated and the tank pressure remained low.