Hey,
I got an Altis 1.6 2015 with 70k on the clock. I experience the similar noise on cold start. Although engine oil is blood to engine but it has got nothing to do with this rattling noise.
1zr-fe is a very common engine globally and many people across globe has registered this issue over the Internet. The most logical I could found was, there is a vvti cam gear which is connected with a rod that goes towards lifters and also timing chain revolves around this gear. As designed by manufacturer, it has a pin lock which keep it stagnant for a few seconds after engine starts. This helps oil to run through the engine before the rotation starts.When that pin malfunctions, this sound originates.
Larger portion of people on forums suggest that this wouldnât affect the engine or its performance. So it all comes down to the noise. if you could make peace with the noise you hear at the start up, there is nothing to be concerned.