i have a groshe adagio hide-away cistern. i think i herited where to start?

mains feed, via service-valve,to hide-away cistern,side- entry- valve , with an exteria push- button on tiled boxing,a air tube working to the central-unit. at the top a over-flow,with a light cross strut-frame in . acoss the centrue holding fast the unit. very limited acess only the size lid ive cut in tiles a horiable hum/buzz noise from over flow .can the cetrue unit be removed top acess what cause this noice, or is it the side entry valve ive order a service kit £20 is this a waste of time ,do i have to buy a hole unit in long run cheaper/time £103 from merchant