What causes plumbing pipes to make a loud hum?

There may be air in your pipes. This is common and can be got rid of by installing 'anti-hammer' devices. - ask at your local Plumbing store.
Air causes noise in water pipes.
This is nothing to be alarmed about, I have had this problem before when doing my rounds and the most possible reason is normally when the water pressure has been turned up either at your water meter or on the councils side in the road. Sometimes when the council does pipe repairs in the road, after switching the water back on they don't turn it back to the original setting and 9 times out of ten they turn the pressure up which would cause your pressure to increase in your pipes creating the noise you hear.