I drove to France once in my old Scirocco, coming back a knocking started, i slowed down, had a look couldn't find anything wrong and continued, it came back, this time worse, i was on the motorway doing 15mph! it was impossible to go any faster the noise was so bad. pulled in to a car park.
shook the wheels to see if there was any play, but none i could notice. tested in the carpark and it seemed okay. continued on my journey slowly going faster and faster and faster, and it never returned again to the day i sold the car.
i can only think that it was something to do with the ball race in the wheel bearing, moving slightly or a foreign object in there, or something, but it went as mysteriously as it had arrived.