There was a strange knocking noise coming from the front of the Mini. I couldn't find the source of this noise until I drove the car with the carpets out. Then I could see the floor around the sub frame mount had cracked.
I was surprised to find a small crack in the tip of the sub frame as well. A little weld fixed that too. There was no rust on the crack, so it must have been caused by some form of metal fatigue.