Intermittent chattering noise under bonnet

[nikcruiser]

So my 100 series started making a chattering noise under the moment intermittently. It’s so random and unpredictable. When it happens it’s loud and then clears within a couple seconds. On a couple of instances the engine light came on and it went into limp mode , however once I pulled up it all cleared. My mechanic can’t identify as it never shows up for him. We checked the ECU and it’s all clear. Has anyone had this issue before? Be nice to narrow it down and get it fixed.