Short thud noise from this area immediately after using flush button

[ElCI]

Hi all.
Tonight I have a new noise from the bathroom toilet. When pressing the flush button there is maybe a 2 second delay and then a short thud that I think is coming from this area highlighted below. Note that the water still appears to be filling so I guess that it's not a valve closing.
Any ideas on this? I'm going to try to take some of this unit apart tomorrow but it looks like the tiling has been done after the toilet has been put in place and may prevent removing of the top plate so it may be harder than it should be

 

[Harry Bloomfield]

When pressing the flush button there is maybe a 2 second delay and then a short thud that I think is coming from this area highlighted below.

Maybe the float dropping, and hitting something on the way down, as the water level drops suddenly.