Thanks for the input guys.
I'm a physicist by trade so thinking about it from that angle I wouldn't be surprised that the roof would act like a big sounding board since that large a sheet of pretty flat metal must resonate a fair amount.
I'm planning on buying 1m x 4xm of this stuff:
https://www.campervaninsulation.co.u...mal-insulation Hopefully that'll be most of the inside of the spare/driver side wheel well, boot floor and underneath the head lining. After that I'll probably buy some more and do the foot wells and up behind the dash as well if I'm not satisfied.
But first, radiator and water pump change. Yay!
Alex