Jolt when releasing brake on autobox

[niteram]

I also have an automatic tourer with electric handbrake (mine is a 1.6 petrol)
the hill start assist activates after the brake pedal is pressed for more than 5 seconds (approx) and will hold the car for about 2 seconds before releasing (enough to take foot off brake pedal and back onto throttle)
This only works on hills, not on the flat - I don't have stop/start
I do feel a bit of a clonk from the back as the brake releases
sometimes when I've forgotten to release the handbrake (after the car is parked) it makes the same noise as it releases, so I guess it's just the way it releases automatically.

but I don't think this is the same as you are describing, it sounds like yours is dropping out of drive then re-engaging as you press the throttle

how many miles has your car done, I had my transmission fluid changed at 50,000 miles due to a leak (but it was very dirty) GM don't specify/reccomend a fluid change mileage as it's sealed for life!!

Rusty, I've never double pulled my h/brake switch - even on my manual car!