Thank you @B51-technician and @Netrange for your replies. I am really struggling with the noise. The inverter is in the garage, which is directly below our bedroom. I have had to severely reduce the charge rate to get to an acceptable noise level. I have improved things quite a bit with this heath-robinson ratchet strap contraption, which reduces the case vibration but it looks awful:
The screwdriver applies a central force to the thin plywood. The yellow squares are the rubber expansion pads that go between the cells.
I want to take the inverter off the wall only once more so I am trying to be well equipped. I will:
a) Put some rubber behind the transformer - trying to find something that has sufficient density.
b) Either mount the unit on 6mm bobbins as you describe - something like these: https://www.amazon.co.uk/Vibration-Absorber-Silentblock-Cylindrical-Compressors/dp/B081PSNCZ5/ref=asc_df_B081PSNCZ5?mcid=e03de42a0b20319da25c80ea2c9fb3a1&tag=googshopuk-21&linkCode=df0&hvadid=697249805889&hvpos=&hvnetw=g&hvrand=17305620947743424505&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1007247&hvtargid=pla-851414358424&gad_source=1&th=1
I am assuming that you bolted straight through the case and didn’t use the supplied support.
Or mount the support on rubber isolation grommets/washers.
Or mount the support/unit to a 3/4" piece of ply, which in turn is screwed to the wall using isolating grommets.
I am not familiar with the rubber plugs in your photo.
This was quite interesting too:
https://www.reddit.com/r/Victron/comments/14d9xh7/how_i_decreased_invertor_humming_victron/