[Thor]

It would be typical for these tanks to be made from tin or lead coated steel.

In the UK, a range of sound proofing materials is available from 'Noise-Killer Acoustics', but it is not cheap.

A relatively cheap way to achieve quite a lot (but not necessarily eleiminating sloshing noises) is to use the foil and bituminous product which builders use for 'flashing' (that is sealing) around things like a vent pipe passing through a roof. This used to be called Flash-Banding, but I am not sure what the current brand name is now. It is available in rolls of various widths. Unfortunately you will get the best effect by using a rather wide piece, carefuly stuck on, eliminating air bubbles, etc. accros the middle of a panel. There is no need to accurately cover the panel right to its edges, but it is worth having a large area covered in one continuous piece.

The mechanical theory is that if the panel is driven to vibrate the small bending of the panel which this entails causes the apparent relative lengths of the panel and the foil to change. Since the foil is relatively stiff in tension, the vibrations cause the panel and foil together to try to squash the soft bituminous material in between. This requires a huge amount of energy, so the panel vibration is very effectivly damped.

I hope that helps.

Thor.