It's a doughnut tank that has replaced the spare wheel in the boot and it has no additional insulation under or around the tank. I'll check to see if anything metallic has fallen down next to the tank, which is vibrating and causing the noise.
I'll also look into adding some sound deadening and/or sound insulation.
Thanks very much.
There should be something underneath the tank if its resting on the boot floor effectively anyway - like this belowhttp://tinleytech.co.uk/shop/lpg-parts/underlay-for-full-toroidal-tank/
Only other thought is if anything could be put ontop of the tank to deaden the noise, though the boot carpet should kill alot of the sound.