Before I start, I have read this thread and it doesn't help Toyota Highlander 2015 rattle sound at (cold) start.
For the past few months, my car started making a rattling/knocking sound (Don't exactly know what to call this sound) when the engine starts. But, it doesn't happen always and there is no specific pattern. It doesn't appear to be a starter motor problem either. Since it is random, the mechanics can't reproduce it.
I am attaching video links to three recordings where you can hear the sound easily.
Please help me understand what could be the problem.