Multiplus5000 PAYG occasional lights flick and brief rattling sound

DIY
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1

Solar installation, three phase, PAYG, only one phase connected and some plugs, intermittent occasional dip in the lights and brief rattling/clicking sound like a brief struggling change over or disconnect-reconnect, happens randomly, not related to load, previous multi1600 installation did the same. what is it?

2

Please post detailed information about your setup, and include photos if applicable.