I started my octavia this morning for the first time in the morning.bought it yesterday afternoon.drove it for 50 miles after buying to get home. No problems what so ever. Started it this morning and it knocked and violently shook the car. Went away after a few seconds and has started perfectly through out the day. Car did shake slightly on idle.

Anyone any idea what's going on.

Next time it does it, dip the clutch straight away.  Does it stop?  If it does, it could be the DMF breaking up.