Vauxhall Insignia banging into gear

Check with Vauxhall if any modified ECU software for your car .There was for earlier models and TSBs on the matter .

You might have coolant ingress into the automatic gearbox oil. This would damage the seals and then further damage the gearbox. IF you have a dipstick for the auto gearbox check if the oil is milky (if yes there is water in it). You can also go to any local automated gearbox specialist and they should be able to diagnose it (usually for free).

I had exactly the same problem (with an Astra though), only after the car warmed up after 15-20 mins, big bang when trying to start from stopped, and also a bang often about a second after I stopped and the car reduced the gear to the lowest one.

Reconditioning the auto gearbox is unfortunately costly (£1000+). You also need to fix the original problem (coolant leaking into the oil) which probably means a rad change. If you are lucky and flush the gearbox a few times with oil you might get away with it, but it's likely that the gearbox is already damaged.