Noise after flush - push button toilet. Easy to fix??

Hi - quick question! My push button toilet makes a loud thump of a noise each time it gets flushed. This tends to be about a minute after the flush... i've taken the back off and can kinda see what's happening (tank gets half full and the thump occurs!) but I've no idea how to go about fixing it... it's not the biggest problem on earth but I'd quite like to have a go and fix it if someone could give me a heads up on what might be causing it.

Many thanks