To get to the boil quickly, a filled jug has to transfer plenty of heat into the water within a short time. That requires a powerful electric element.
This noise reduces as the water heats, because the bubbles stop collapsing (instead they start forming the visible bubbles that you can see).
The problem is often worse in areas where the water is “hard” (it has a higher concentration of minerals). The furry scale that the minerals leave on the element seems to make the noise problem worse.
You can descale your jug or kettle by covering the element (or the bottom of the jug if it has a concealed element) with a 50:50 mix of white vinegar and water – use just enough to cover it. Do not boil the jug. Leave the solution to soak for an hour or 2, shaking occasionally. Then rinse the jug thoroughly.