Big thump/bang when hard shifting into second gear
 

Did my first santa pod runs in the classic wagon on saturday. When you shift into second gear and engage the clutch something goes 'WHACK' :brickwall as if you have been shunted up the arse like in a bumper car or something! Pulling off the line its fine and all other gears seam fine but 1st to 2nd is sounding like something bad is happening. Any common weak areas I should be looking at? Mounts of some sort perhaps or is this normal behaviour