Martin1969,
As for the outbound flight, it does very much soud like the autobrake kicking in.... if you had been on the ground for a few seconds and it happened, it would have been MED autobrake which can be cause quite a sudden jolt as it starts to take effect.
However, when LO autobrake is used, it doesn't kick in until around 100kts as the aircraft is slowing down but is usually a lot smoother than MED autobrake. The decision whether to use MED or LO is upto the pilots and will be affected by a number of factors such aircraft weight, headwind and which taxiway they plan to use to exit the runway.
Sometimes, as it sems in your case, if the pilots touch down a little further down the runway than expected (e.g. due to a slight tailwind) and using LO autobrake, the pilots need to brake manually in order to make the required runway turn off point. This is done by manually presing the brake pedals with their feet. Like in cars, some aircraft are more sensitive to pressure on the brakes than others so it can sometimes result in a bit of a jolt. As mentioned before, the bang most probably was a toilet seat coming down!
As for the inbound flight, that throws up an interesting topic. The Captain's prime responsibility is for the safety and peace of mind of all passengers and crew on board the aircraft, including their own. In situations like this, it can be very difficult to find the right thing to say to please everyone.
I agree that sometimes it is better just to say "There was a minor technical problem but it's now fixed" but sometimes one passenger can get a whiff of what the problem is e.g. burst fuel pipe on the ground sprayed the aircraft, they tell the person next to them, who in the queue for McDonalds tells another passenger that the plane burst a fuel pipe. That person gets on the aeroplane and tells the passenger next to them that the aircraft has a fuel leak and before you know it, there are all kinds of versions of what the actual problem is.
So in this scenario, should the Captain tell the passengers "There was a minor technical problem but it's now fixed" and have the passengers believe the story they have heard or should the Captain dispell any rumours going around and explain the situation exactly as it is.... you chose!
Hope this helps!
SJ777