The noise is presumably due to the tank filling when it drains when hot or cold water runs. The sound could be due to water hitting the existing water in the tank as it fills; vibration in pipes as water turns on and off (water hammer) or sound from the valve itself as water exits. Sometimes pipes can bang against each other so clipping them properly so they can't move may help (or sliding small sections of insulation onto them to separate them).
Replacing a section of copper tubing where it enters the tank with about a meter of PEX tubing may also help. PEX tends to give more of a cushioning effect than copper.
The jet in the valve or the valve itself may be a low-pressure type that's primarily used for filling tanks rapidly (e.g., agricultural water troughs). It may be possible to replace the jet in the valve (or maybe the whole valve would have to be replaced) with a high-pressure type that fills the tank more slowly.
Have a look at these links that discuss the problem in a little more detail: