Without a video with the top cover off during it happening i would initially suspect anti vac valve needs cleaning or for the low cost replacing .
To me, if you are removing it to have a look id replace it, assuming thats what it seems to be.
Its basically a consumable part with a long life!!!
Ade Smith. ACS Evo Leva v2. Kafatek Flat max 2, ssw 2024. Mazzer Philos, Craig Lyn HG-1 prime., WW key mk1.