Hi, since last week, everytime I change into 1st 2nd or 3rd gear and lift up the clutch really quickly there is a thud or a knock which comes from right under my feet and the car judders with the knock. It doesn't happen at all if I lift the clutch normally or smoothly. Any idea what this could be?

maybe a gearbox mount?

try the engine mount under your battery... mine did this other week once i tightened it it was fine