Gear stick knocking when engine cool

Over the last few months I have noticed that there is a very noticeable knock being delivered through the gear stick of my 2001 Octavia 2.0 (70,000miles).

The conditions under which you hear and feel the kick are as follows:

1. Move off from stand still (e.g. lights, traffic queue),
   
2. Work your way through two or three gears.
   
3. You see the next set of lights/stationary car before you've really got to a cruising speed, so you take your foot off the accelerator, but do not brake just yet
   
4. Just as the car starts to slow (i.e. the engine no longer pushes it) there is a knock that you can hear and feel through the gear stick and accelerator pedal.
   

This only happens while the engine is cool (and may be more noticeable because it's been really cold of late). Once the engine is thoroughly warmed there is no knocking.

My questions are these:

1. What do you think could be causing this effect?
   
   1. 
      
   2. something as mundane as using the wrong kind of oil (last top up I used Halfords slightly more expensive recommendation rather than the exact spec as laid down in the owners manual)
      

[*]Is it indicative of something serious that is going to go very badly wrong at some random point in the near future without any more warning? (in which case I'll plan to sort the problem out rather than face the chaos of the car being completely out of action for a few days when it's least likely to be convenient.)

Cheers!