Slight knock after gear change. 1st to 2nd

[182.]

As title really. sometimes when i change to 2nd (from 1st) there is a slight knock and it sounds like its coming from the rear of the car.

any ideas of what it could be?

 

[Hughes1990]

Gear linkage hitting the exhuast maybe? mine hits the decat pipe when I put it in reverse

 

[182.]

i dont have a decat but i do have a cat-back exhaust. if that makes it any worse?

could it be some sort of mount becoming weak?