easy peasy
I expect it has a ballcock in it.
It is worn out.
If you feel like it, you can take it off, dismantle it, clean it, fit a new cone and washer, reassemble it, refit it.
Or for a few pounds more, you can take it off and fit a new one.
You can get modern "silent" valves now which are extremely quiet. Old plumbers often prefer the big brass ones.
You can fit it yourself, once you know where the main stopcock is and have bought a set of adjustable spanners, but any competent local plumber over the age of 14 will know how. Ask friends and neighbours for a recommendation. He should wear blue overalls and have black fingernails.
You must get a "type 2" valve which has the little plastic bridge on the top. see here.
The Pegler Prestex is about ten times as good as the cheap ones but only costs twice as much. Read the review comments.
If you like jigsaws or tinkering you can repair the old one at your leisure on the kitchen table, and wrap it in a plastic bag. Put it next to the tank ready to swap in ten years when your new valve is worn.
People who blush to say "ballcocks" can call it a float valve.