Why is my fuse board buzzing when I'm using the shower?
Any time you have buzzing coming out of a fuse panel or breaker panel, you need to get a qualified electrician in immediately to find out what's going on. 

These things contain large buss bars and depending on your local electrical code, unfused power entry. Any minor loss of connection integrity that causes a loose connection anywhere whether it's inside a breaker, where the breaker/fuse attaches to the buss bars, broken buss bar insulation stand-offs or in the cable connection to the bars can unleash an arc in the 8000-10000 degree F range that burns through metal and instantly sets fire to any wood surrounds that heat gets transmitted into.

The best demonstration of the power of 220V (which did not blow the 50A range fuse) was a cook stove that had a weak spot develop in the element. It glowed bright orange in the break, burst into arc and burnt through the muffin tin my mother threw on it to contain the arc flame. The ion plume reached the ceiling and scorched a 3" dia area black. A water heater pulls enough current to cause such an arc wherever there's a loose connection.