The petrol engines have hydraulic valve adjusters, or tappets, and can "rattle" until the oil pressure builds up. They can take a few seconds before they work correctly. It is more noticable if the engine hasn't been started for some time, could be the case if the ones you have viewed have been on garage forecourts some time. With day to day use I would doubt if you would notice any sound at all, I don't with mine. If the noise persists I would look at another vehicle as it could mean the hydraulic tappets or even the camshafts need changing. Help this helps.