I have a multiplus II 10kva inverter setup for ess, when the grid is switched on after a grid outage, the internal big relay/contractor makes a loud rattling sound, like its switching on and off very quickly, this lasts only about a second or so. Is this normal or is my inverter behaving strangely?
From the old forums:
I have a 3 phase victron system (MP 5000) that works as expected. When a grid failure is detected there is about a single “click” and the MPs are running offgrid. That seems fine, but what i’m not sure about:
When grid connectivity is restored, the MPs are starting a “Relais-celebration”, toggling something with about 20Hz for at least 3-5 seconds.
Is that “normal” behaviour, something like a must-do functionality test of all relais, some sort of “soft-connect” to precharge some capacitors, or is this more an Issue that needs to be looked at?
Example
https://static.equinox-solutions.de/Victron/clicki-click.mp4
Answer:
100% normal. Since you have 3 systems, it is even more noticeable
https://community.victronenergy.com/questions/314447/relais-when-restoring-grid.html
Thats exactly the sound im experiencing. I thought my MP had a fault, but glad its normal behaviour now
There are three switching. So yeah it can sound pretty busy. 
See the block diagram here. Two for the input and the ground relay.